I need to use paypal's IPN to create an order form on a client's website. He creates kydex holsters for his customers, so I need to have multiple options on the order form. In addition, I need to send an email containing an invoice to the customers, as well as my client, after the transaction is completed. I have literally ZERO experience with paypal IPN so I am looking for any advice, guidance, or examples anyone here has to offer me. I haven't had too much luck googling.
There are two really good options for this which don't include you doing it by hand. Why reinvent the wheel? Lots of really good, free examples of this exist already.
Use a CMS with e-commerce platform (drupal + ubercart or other, wordpress + woocommerce or other, magento) which include custom PayPal ordering.
The way PayPal recommends we do this is to generate a very complex, complete button on their website and then just snap this into a client site via php copy -> paste. This is a pretty sure-fire (pun) option for payment + options + invoicing, direct from PayPal.
Unless you're quick with POST, you'll be slogging for weeks on something that's been mastered and given back to the community several times.