For example, if a customer paid $100 for the amount of bitcoin needed to buy my product,, how much would I get in dollars after I have exchanged her bitcoins to my local currency (use any currency and today's current - or BitPay's - bid and offer prices to provide an answer).
If the customer has dollars and you wish to receive the same currency then there will probably be faster, cheaper, more mainstream options out there than using Bitcoin as an intermediary. The fees are low but, as you note, the spread is a pain.
Bitcoin really makes more sense when the customer has bitcoins and/or you wish to receive bitcoins. Additionally, Bitcoin can sometimes be pretty efficient if your customer's local currency is different to your own (in which case you'd have to deal with an FX spead at some point anyway).
Now I have to investigate the best service for providing this immediate exchange facility. Is there a list of the possible options?
The company BitPay your referenced is one option. Acting as a payment gateway for merchants to receive local currency while customers shop with bitcoin is their main business.