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".
Most merchant woftware creates a new address at the checkout and waits until the total confirmed amount received at this address is at least as big as the cost of the order. Sending 0.0999 would just make the merchant wait until you send another 0.01.