Hey, great question! The relays you subscribe to are the relays you read and write to. If you have added a paid relay to your feed, but not paid for the relay you will likely have read permissions on that relay (you can see posts from others) but not write (you won’t be able to publish posts to that relay).
The key is to get some really good relays. IMO nostr.wine and filter.nostr.wine are a great place to start and offer great connectivity to nostr with minimal effort.
Tagging some experts here too. Hope this helps! #[2]