Inventory Add Item error

So I wrote an inventory script, based on a list to store my items in. I've made a function to add the item, where I pass in an id, and an amount. In the function I check multiple things. First I check if I even have anything in my inventory, if I don't, it's easy. I just add the given item as the very first item in my list. Second I check if I already have the item in my inventory and if it's stackable. I do that with a bool function:

 bool doesExist(int id)
     {
         for(int i = 0; i < inventoryCount; i++)
         {
             if(inventory[i].itemID == id)
             {
                 existingSlot = i;
                 return true;
             }
         }
         //existingSlot = inventoryCount -= 1;
         //Reset inventoryCount
         return false;
     }

If I already have one of these items in my inventory and it's stackable, I just add the given amt to the total amount for that inventory slot. If however, I do not have this item in my inventory, or it's just not stackable, I have an else statement. In the else statement I add a new item with the given id and amount. It all works great.

But... I made a crafting system, where I find a recipe (premade), and then I loop through my inventory to check if I have all the item. If I do, I call the addItem function. This all works great the first time, but the second time I try to craft, it adds the item with the id one below (so say if I craft an axe with the id of 4, second time I craft the axe, it adds item with id of 3)

Crafting System: public void SelectRecipe(string name) { for(int i = 0; i < recipes.recipes.Length; i++) { if(recipes.recipes[i].name == name) { selectedRecipe = recipes.recipes[i].recipe; index = i; //print(selectedRecipe[0]); CheckforItems(); break; } } }

     void CheckforItems()
     {
         //print("Checking for items");
         //print(inv.inventory[inv.inventoryCount-=1].itemName);
 
         string[] first = selectedRecipe[0].Split(';');
         string[] second = selectedRecipe[1].Split(';');
         string[] third = selectedRecipe[2].Split(';');
 
 
         for (int i = 0;i < inv.inventoryCount; i++)
         {
 
             if (inv.inventory[i].itemName == first[0] && inv.inventory[i].itemAmt >= int.Parse(first[1]))
             {
                 firstSlot = i;
                 requiredFirst = true;
             }
 
             if (requiredFirst && inv.inventory[i].itemName == second[0] && inv.inventory[i].itemAmt >= int.Parse(second[1]))
             {
                 secondSlot = i;
                 requiredSecond = true;
             }
             if(requiredSecond && inv.inventory[i].itemName == third[0] && inv.inventory[i].itemAmt >= int.Parse(third[1]))
             {
                 //Remove first item(s)
                 inv.inventory[firstSlot].itemAmt -= int.Parse(first[1]);
                 //inv.slots[firstSlot].transform.GetChild(0).transform.GetChild(0).GetComponent<Text>().text = inv.inventory[firstSlot].itemAmt.ToString();
 
                 //Remove second item(s)
                 inv.inventory[secondSlot].itemAmt -= int.Parse(second[1]);
                 //inv.slots[secondSlot].transform.GetChild(0).transform.GetChild(0).GetComponent<Text>().text = inv.inventory[secondSlot].itemAmt.ToString();
 
                 //Remove third item(s)
                 inv.inventory[i].itemAmt -= int.Parse(third[1]);
                 //inv.slots[i].transform.GetChild(0).transform.GetChild(0).GetComponent<Text>().text = inv.inventory[i].itemAmt.ToString();
 
                 //Add Item to inventory
                 AddItemToInv(recipes.recipes[index].id);
                 //print(recipes.recipes[index].id);
             }
         }
     }
 
 
 
 
     void AddItemToInv(int id)
     {
         //print(id);
         inv.AddItem(id, 1);
 
         requiredFirst = false;
         requiredSecond = false;
         firstSlot = 0;
         secondSlot = 0;
         thirdSlot = 0;
     }

Inventory addItem function: public void AddItem(int id, int amt) {

         //If we don't have anything in our inventory
         if(inventory.Count == 0)
         {
             inventory.Add(database.items[id -= 1]);
             inventory[0].itemAmt = amt;
 
             //Visual
             slots.Add(Instantiate(slotPrefab));
             slots[0].transform.SetParent(inventorySlots.transform);
 
             GameObject itemObj = Instantiate(itemPrefab);
             itemObj.transform.SetParent(slots[0].transform);
             itemObj.GetComponent<Image>().sprite = database.items[id].itemIcon;
             //Reset id
             id = 0;
             //print(id);
 
             itemObj.transform.GetChild(0).GetComponent<Text>().text = inventory[0].itemAmt.ToString();
 
             return;
         }
 
         //If we already have one of these items in our inventory
         if (doesExist(id) && database.items[id-=1].stackAble)
         {
             print(id);
             inventory[existingSlot].itemAmt += amt;
 
             //Visual
             slots[existingSlot].transform.GetChild(0).GetChild(0).GetComponent<Text>().text = inventory[existingSlot].itemAmt.ToString();
 
             id = 0;
 
             return;
         }
         else
         {
             //print(id);
             inventory.Add(database.items[id -= 1]);
             //Reset id
             //id += 1;
             inventory[inventoryCount].itemAmt = amt;
             //print(database.items[id].itemName);
 
             //Visual
             slots.Add(Instantiate(slotPrefab));
             slots[inventoryCount].transform.SetParent(inventorySlots.transform);
 
             GameObject itemObj = Instantiate(itemPrefab);
             itemObj.transform.SetParent(slots[inventoryCount].transform);
             itemObj.GetComponent<Image>().sprite = database.items[id].itemIcon;
             //print(id);
             //Reset id
             id = 0;
 
             itemObj.transform.GetChild(0).GetComponent<Text>().text = inventory[inventoryCount].itemAmt.ToString();
 
             return;
         }
     }

The error seems to be located in the last part of the addItem function. The id changes, and I have no idea why. Can anyone help me pls? :3