Many merchants offer to save credit card info in an encrypted form for faster checkouts from their store in the future. How is it that merchants can store this information in an encrypted format and still be able to decrypt it to plain text later?
I think you are confusing encryption with hashing because hashing is one way whereas encryption is two-way providing you have the proper key to decrypt whatever is encrypted.
Short answer is: they are storing the key somewhere (in a hopefully secure location). Whenever they need to decrypt the credit card number they hand the encrypted data to a module that looks up the key, decrypts the card number, and returns the card number to the payment system.