Paypal setup and Subscription Details

Dear M'ship Team - I'm somewhat confused. I have read all the details I can find about setting this up, and I finally do have it working, BUT...

1 - It always treats an payment as a subscription, even though I want there to be a single payment and that's it (about half the time) and the rest of the time, I want it to take one payment, and then a later balance payment. I may have a regular subscription requirement as well, but let me tell you, this should be a whole lot easier than it is. If I make the subscription indefinite - should't just work as though they NEVER renew? Here is the text for this option in the subscription panel:
Mode details
Finite - user remains at this level for a set period of time before ending

Indefinite - user remains at this level for ever.

Serial - user remains at this level for a set period of time and is then renewed at the same level

Note: - depending on the payment gateway used, changing the price will not alter subscriptions charged to existing members

2 - I finally got this working using the Paypal single gateway - even though I set it up exactly the same way as I had for the Express Gateway - BUT ... the Express gateway did not work for some reason - it came back to a page that was protected, and did not update the user's subscription level.

Any clues as to why that might have occurred?