How to calculate dates depending on certain fields?

I am looking for a way to calculate a due date in the Invoices module depending on the value in the dropdown/select "Payment Term" (in this dropdown I have the values 30 days, 50 days and 60 days).

So the thing is, I added a formula field in the Invoices module and used the following formula to calculate the due date:

Datepart(Adddate(${Invoices.Invoice_Date},30,'DAY'))

This will output a new date (invoice date plus 30 days). That works fine! But now I want to use something like that:

If Dropdown "Payment Term" = "30 days" then add 30 days to the Invoice Date

If Dropdown "Payment Term" = "50 days" then add 50 days to the Invoice Date

If Dropdown "Payment Term" = "50 days" then add 60 days to the Invoice Date

Is this possible?

Many thanks for help.