I have a program running in an environment that supports URLs, but no javascript.
Say it's a Word document (though it's not).
Is there a way to craft the PayPal payment URL such that it's all that I need, without the requirement of an HTML form or Javascript?
http://paypal.me basically exists for simple things
You can create a Buy Now button via https://www.paypal.com/buttons with NO dropdown menus or text fields (which require HTML) and then, once you've generated your button code, clicking on the "Email" link above will give you a URL.
If you also want the URL to have editable fields, then before you create the button, in Step 2 uncheck the option to save the button at PayPal, generate the code, and click the link to remove code protection. Variables are documented here: https://developer.paypal.com/docs/paypal-payments-standard/integration-guide/Appx-websitestandard-htmlvariables/#technical-variables