redirect with data included after signing

I have a remote computer service business with 3 websites.

What I need is this: When a customer on 1 of those sites chooses a service plan, they are either directed to a page on our site with the support agreement or sent a link to your site with the support agreement or emailed similarly. Once they sign the agreement, which contains the name of the site they were on, the plan chosen, the price of it, and the number of computers supported by the plan, as well as other usual info... they must be sent to a payment page on my main site. That page should receive those 4 fields in the redirect so I can grab the data with $_GET or $_POST or $_REQUEST in php and populate the payment form.

Docusign claims to have that feature and it seemed to be relatively easy to set up; however, after 4 hours and 3 support calls yesterday, they could not get it to work.

If you can do that and it's easy for me to use, then you get my business.

FYI, the site name and plan name fields on the support agreement need to be drop-downs ideally.