Conditional OSlogin

We are selling Map Interpretations. I have setup OScommerce from our shopping site such, that we can sell a) 1 Map Interpretation for $10 b) 5 Map Interpretations for $40 c) 10 Map Interpretations for $70 1) After the customer has bought lets say 5 Map Interpretations he/she should receive an email confirming this. This should be easy, I think. 2) When the customer goes to our Map site (which is different from the shopping site) he should be able to redeem the Interpretations he/she has bought. Thus, we want the customer to login from outside the shopping (with the Oscommerce login and password). If he cannot login, there should be an error message telling that he/she should go to the shopping site first and create an account. After login the customer can see that he still has 5 Map Interpretations to redeem. After that he is able to send us his map for interpretation via formmail attachment. If he has no current Map Interpretations to redeem he will not be able to send us the map. The formmail site should only open if the customer has Map Interpretations to redeem.