Buy a billing system, it'll save engineering hours having to deal with it and if you choose right they'll scale with your company.
For example if your homebrew thing doesn't handle price changes, that might take you quite some time to build in functionality. That time spent is time where you couldn't quickly react to market changes, which is money left on the table. Had you bought something that supports a simple use case like "change the price", you'd hit a couple buttons and boom... your product now sells for a different price.
A companies billing system is one of the most important systems in the company. It is literally how money gets into your pocket. If you build it yourself, you will sign up for pissing away a ton of time writing code that literally will let you collect more money from your customers. Had you bought something, you'd just fucking go do the change right away and collect more money from your customers almost instantly.