Product registration via sign up form

Hi community   wondering if anyone’s ever done anything similar. 

We sell d2c as well as through some wholesalers. For our customers that purchase direct from us, we have a great service program that they enter into that after a certain amount of time after purchase, Klaviyo emails them info about our repairs and other things. We’d love to offer something similar for wholesale customers.

An obvious solution was a dedicated embedded sign up form where customers would follow a QR code included with their purchase to this sign up form, where we’d then enter them into a flow. 

However, we’re stuck on the scenario where a customer purchases subsequent items and follows the code to the form again - we don’t want them to leave their current flow (if they’re still in it - the flow can be 2-3 years in length as they’re not throwaway items and we promote in-house repairs), but we want their new item to receive the same. 

The only thing I can think of is if they re-entered the flow by adding a + to their email (ie, abi+2@example.com) to register them as another profile, but that’s not ideal from a customer experience perspective. Has anyone done anything similar and share some advice?

Thanks in advance!

Abi