When prompted with a BUY NOW page, indicating 0.1 BTC are to be sent to whatever address... how do you proceed?
Do you send 0.1 + transaction fee, or 0.999 BTC?
You send 0.1 to the retailer. That is the payment to them. Then, if your transaction requires a transaction fee, you include that as well.
Will the retailer fuss over a transaction fee amount missing?
Most retailers use automated software that is looking for a specific payment. If you don't send the correct amount, the software won't recognize it as being "paid in full".