Just so I understand correctly...

They sign up for your list and then on the thank you page you suggest they buy the product?

If so, I probably wouldn't worry too much about that. As Graham says, take your time and build a relationship with these people.

Maybe invite them to a private Facebook Group you run. And every so often put a link to a product you think they'd like.

If they trust you (which takes time) they'll take your suggestions.