Custom Variables Passed to PayPal

I'm working on a setting up a new PayPal Express link and I'm wondering how I can pass a custom variable from the checkout to PayPal when the form is submitted. My situation is that we give a portion of our profits to charity and we let the individual select (radio button) which charity. I can probably figure out the radio button form option, but the passing of that variable on the "post" submit is where I'm stumped. I know PayPal has both the "on0/os0/on1/os1" fields as well as the "custom/invoice" pass-through variables. Is Zen Cart already using those to pass info? If not, how can I access them.

On a similar note, I noticed that the "Special Instructions or Order Comments" Zen Cart field doesn't get pushed to PayPal anywhere. Is it possible to put that in PayPal's "Description" field instead of the default "Shopping Cart"?

Thanks in advance for any help.

All the information for any attributes you configure to products on your store will be shown in the order details submitted to PayPal, AND they will be visible to both you and the customer in the order history after the order is completed.
There's no need to use "custom variables" to do what you're asking for. It's already built-in.

That I get, but I guess I'm in the habit of having all the info in PayPal, not in the shopping cart. That's something I'll have to get used to.

But the radio button issue is something completely different. That is a value that I need to pass somewhere. Whether it goes to the order info in the Admin panel, or off to PayPal. So I guess this isn't a PayPal issue after all, but more a general Zen Cart issue, huh...

Yes. If you want to learn how to set up Attributes for your products, there's an starting FAQ for that here: https://www.zen-cart.com/tutorials/index.php?article=57