Q: I've bought an album and saw afterward that there was a collection...

Not unless there is something mentioned on the collection's page in the store, which is probably doubtful. You could try contacting iTunes Support and see if they will refund or credit you so that you can purchase the collection, but purchases are considered final : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption