If_Else Calculation

Apologies if this has been answered already but I couldn't find exactly what I was looking for in the previous posts. I'm trying to add a discount to a calculation or use an if_else statement to return a value.

I'd like to figure out if there is a way to use conditional logic in calculations? For example, I have products that can be purchased in my form and if the total of those purchases add up to $21 or $30 I'd like to subtract $3 from the total. Initially I was trying to do this in a single calculation field where the total was adding the Saved Values from a few fields together and I wanted to apply the discount in that same field, but this has been proving to be a bit more difficult. So instead, I have a subtotal field and based on that subtotal field, I've added a 2nd field for total and would like to subtract an amount of $3, $6, $9, etc. based on whether or not the subtotal value is equal to specific pricing, or else return the subtotal value in the 2nd field.

Hopefully this makes sense? Any assistance would be appreciated as I believe my previous discussion topic was removed 🙁