Thanks for the heads up on this request for support.
Whenever tax may apply, s2Member Pro Forms will collect some location-specific details that need to be considered before your Pro Tax Configuration is parsed (i.e. before sending the customer to Express Checkout). Since pro tax calculations can include location-specific details, s2Member Pro forms need to know where your customer is located, before sending the customer to Express Checkout.
As Cristian mentioned, Express Checkout is a bit different from PayPal Standard. It’s up to s2Member Pro, to calculate the tax (as you’ve configured with s2Member), because PayPal Standard tax calculations do NOT get applied during Express Checkout (e.g. it’s up to your shopping cart system – s2Member Pro).
While I can understand your desire to reduce these fields when you’re running only a flat percentage rate, it’s not something s2Member Pro is capable of in the current release. That is, the location-specific fields will always appear, if tax may apply. If you empty your tax configuration file for s2Member Pro, and add the flat rate to the total cost, they will dissapear though.
Or, you could try implementing your own custom Pro Form template if you prefer.
Pre Sale FAQs » Is it possible to modify s2Member® Pro Form templates?