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

Shopify Apps
1

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