I haven't found the exact answer I'[m looking for on the post office website, but here is what I did find: stamps and meter strips can be combined (I believe the electronic postage we are using is considered metered mail).

Postage Questions

Can I Use different postage types on a mailpiece?

Postage can be handled in a variety of different ways. If necessary, the following types of postage can be combined on a mailpiece and used as valid postage to meet the minimum required postage amount for the item:
Postage Meter Strips
Stamps
Pre-printed postal stationary.

Also, if a mailpiece is taken to a Post Office to pay for postage, the customer pays the Retail postage price even if Delivery or Signature Confirmation was acquired at the Electronic price

This does not fully answer Robert's concern about discounts, except that if you take it to the window (something they are trying to avoid with on-line postage products), you will end up paying the full price without any discounts.