According to the PayPal documentation, when you are using IPN, after their software sends information to the secure URL that you specify, you need to respond to a secure URL that they provide.
Setting up the secure URL to provide them is no problem; there's lots of documentation on that. And I've done some programming where I fetch files in Perl via http. But the whole security certificate thing for clients has me confused.
Thawte has instructions for installing developer certificates in "Apple Codesigning", "Marimba Channel Signing", "Microsoft Authenticode", "Netscape Object Signing", "Microsoft Office", and "SunJava". No instructions for installing a certificate in Perl/LWP.
And I'm a little apprehensive about using LWP anyway, for a financial application - the documentation says the library does not support multiple simultaneous requests. All I need is for users to send money without my site knowing about it, simply because LWP can only juggle one user at a time.
If you expect 15 customers a month, at $5 or $10 each, you can't afford to have a bank card account; your service fees are going to exceed that. If you ask customers to find a stamp, find an envelope, address it, write a check and put it in the envelope and remember to mail it, and return in 2 weeks for electronic services, you're going to be lucky to have 1 customer a month. And if more than one check in 7 bounces, you're better off burning the checks than depositing them.
There is very little documentation at the c2it site. It might be a way to email money to your cousin, but it doesn't look like there's any way for a website to accept money and immediately deliver electronic goods or services.