We have an existing website & android app with thousands of courses. Which will keep increasing. Considering the fact we can max add 10K products in IAP, so I started to look for what others are doing in this case. So after reading some articles, I found out I can set up courses in tiers. So I have created 87 tiers which are non-consumable. Now the issue is when I was testing, payment system allowing users to purchase only once to a tier. next time it is showing that you have already enrolled in this product (~something like this). Payment system not allowing the purchase of the same product_id again. Now after searching lots of articles, I didn't find any solution.
