PayPal php和WordPress

Question

A few years ago I created a test web site that included the files from this tutorial - PayPal Express Checkout with PHP by Sanwebe, although I only used it in Sandbox mode it seemed to work fine.

Recently, I decided to revisit the project, though this time around develop it using WordPress (Version 4.5.3), and ran into problems.

In the original site I put all the files into the root directory then kicked off the process by using -

<form method="post" action="process.php?paypal=checkout">
  <input type="hidden" name="itemname" value="Product One" /> 
  <input type="hidden" name="itemnumber" value="1" /> 
  <input type="hidden" name="itemprice" value="1.50" />
  <input type="hidden" name="itemQty" value="1" /> 
  <input class="button medium green" type="submit"    name="submitbutt" value="Download " />
</form>

in the product page. The action call to process.php?paypal=checkoutlooked up the process.php file in the root folder and carried the paypal=checkout function, great.

However, using the same form in a WordPress product page fails, as it tries to find http://website.com/product/process.php?paypal=checkout rather than looking up the process.php file.

After spending a couple of weeks searching Stack Overflow and WordPress Support, as well as general web searches, I'm stuck.

So far I have tried -

1. Including the files in the root directory as before - didn't work.
2. Including the files in the theme directory as before - didn't work.
3. Including the files in the child-theme directory as before - didn't work.
4. Tried using get_template_part() - didn't work.
5. Tried using include('path/to/folder/my_script.php');, include_once(), require()
6. Tried using include (TEMPLATEPATH . '/mypage.php');
7. Tried adding the process.php into both the theme and child-theme functions.php files
8. I even tried creating a Plugin, but that also failed, maily because I'm rubbish.

Any assistance would be greatly appreciated.

Thanks

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