How do I make a Paypal shopping cart using a checkbox form and PHP?

[SaW nO.3]

All I need is for someone to simply tell me what to do. You can't google stuff about Paypal, or even go directly to their website and read their docs. There are too many different outdated answers and versions of Paypal that it's impossible to learn it right now.I am trying to make a website that sells puzzles in PDF format. The website has username/password login so if someone buys a puzzle they can login and see the products they bought, etc. I decided the best way to make this was to integrate my website with Paypal.Right now there are only 4 puzzles, and they are very cheap. A full-fledged shopping cart is overkill but separate "Buy Now" buttons are also bad. I want a simple list where the logged-in user can check off which puzzles they want to buy in a checkbox form and then click a button that says "Buy Now" which will lead to Paypal. Then after completion of the payment, the user will be sent back to my website.I'm pretty sure Paypal has shopping cart features but I'm afraid I am going to spend hours and hours reading about the wrong thing before realizing it (either an outdated Paypal feature or a Paypal feature that is named ambiguously).Here is what I did before, and it worked, but it was clearly not how it was supposed to be done. I simply went to the Merchant Services and added a regular "Buy Now" button to my form. I stuffed information I needed into the "custom" field to make a system resembling what I wanted, and from that, manipulated the "item name" field. "Buy Now" buttons are intended for selling just one product, so the receipt on Paypal only shows one product name, but I wrote code such that it would know from the custom field how many products the user was buying, and would display something like "3 Items: Puzzle 1, Puzzle 2, Puzzle 4". I think at one time it might have been an option to sell mutliple products but I'm not so sure anymore. I can't do this anymore because when Paypal changed, it took away the ability to make a Sandbox "Buy Now" button, even if I switch to classic. If I click "Merchant Services" in the sandbox, it simply doesn't take me there anymore. I know there is IPN testing but there was a dreaded day a few weeks ago when I was testing for hours with the IPN tester, and it wasn't working, until I just made a real button and suddenly it worked. So I don't trust the IPN tester thing in the dev tools anymore.I guess this would be a good time for me to just ask the general public how to do this the right way. Is there some sort of Paypal service called "Shopping express cart express cart standard pro standard cart" that does exactly what the ** I want it to do?Thanks!