Paypal Sandbox questions

I have two products configured - a six month membership and a 12 month membership. Both are configured to have 1 day free, and then charge for the remainder of the term.

I have just gotten everything to point of testing payment via PayPal Sandbox. Here's what I see.

1. When I choose a membership (I have only tested against the 6 month term), I receive an email telling me to click a link to complete the process. This is correct.

2. When I click the link, I am redirected to PayPal to complete the purchase.
2a. How is the "1 day free" supposed to work, if the payment process is immediate?

2b. The amount of the payment on the PayPal site is blank. It askes me to fill in the amount, and perform an update calculation. To this point I have only filled in the correct amount. But is this normal operation? Shouldn't the price I configured be pre-filled-in? What if someone enters an incorrect amount? (which I will do once I get a correct amount processed correctly)

3. When I have updated the amount, I can proceed to complete the purchase, which goes through OK. BUT at least as I understand it, the Paypal Sandbox doesn't really do IPN. Instead, it provides an "IPN Simulator," which requires/allows you to provide the necessary field information for the sale. I'm having some difficulties with this - in particular, determining the correct Transaction ID (txn_id). I've tried using the txn_id that PayPal provides in its completion screen, made up ones, blanks, etc., but I get a consistent error from aMember: "paypal_r ERROR: finish_waiting_payment error: Payment not found with such ID#".

3a. Can anybody help with how to determine aMember's ID#? And should that be used as the txn_id?

3b. I see some potential additional problems in the transaction from PayPal as well, depending on what aMember uses to try to match things up - but I may have not made it far enough through aMember's processing yet to have hit them. For one example, my products are configured with spaces in their names, but PayPal is concentenating strings in the item_name fields.

3c. There could be other similar kinds of potential mismatches. One more potential example: my products are configured with a price (of course). The transaction I'm putting through PayPal is for that amount. The transaction confirmation coming back from PayPal has the mc_gross field set to that amount, the mc_fee field set to their processing fee amount, and mc_gross1 is set to the difference (since I'm electronic, there are no shipping, handling or tax amounts). BUT, the mc_gross1 field, which is my net proceeds, does NOT match the product price (because of the mc_fee). Is aMember going to complain about that?

Thanks, everyone. Sorry if I'm rehashing old questions that have already been asked and answered.