What's the best eCommerce system for custom coded plugins?

[usrec]

Seeking ECommerce System Recommendations for Plugin and/or Custom Coding

Have a site that produces a filled PDF from a webform.

Needs:
1) Charge BUYER-A $X per form (1 Payment = 1 Form Access).
2) Sends Email for Form Access (With Code) to BUYER-A's Client to Complete Form.
3) Track when Form is "Completed and PDF Sent".
4) BUYER-A can purchase multiple Web Form Access's/USES/Gift Card's
5) Need Commerce/Payment system to track and manage BUYER-A access uses and ability to send to another Form Access to additional clients.

In Dev mode at srpds.com utilizing:

CF7
CF7 Multi-Step Forms
Contact Form 7 Multi-step Pro
PDF Forms Filler for Contact Form 7