Hi adyleroux.
I think your basic issue here is that you want "this cell" to contain both a fixed value and a formula which will add the value in a second cell to 'this cell's' value under a specific condition.
Cells can contain an entered value, or can contain a forwula which calculates a value, but cannot contain both.
If a cell contains a fixed value, then placing a formula in that cell replaces the fixed value with the formula.
If a cell contains a formula, then entering a fixed vlue in that cell replaces the fornula.
To make this work, you need to place the fixed value in a separate cell or write it into the formula itself.
Here are two examples.
The first example contains the base amount in C2, and the amount to be added to that amount in C3.
The Popup menu is in cell A2, and the formula calculating the result is in B2
The second example uses the same cells for the popup menu and the result, but contains the base value and the bonus for Ady in the formula.
The formula inB2 of each example is shown below its table.
Regards,
Barry