Here is how to store extra item:
1 Create a mule/alt
2 Mail any/all additional items to mule. DO NOT open the mail on the alt, leave it unopened.
3 When you need the items back, instead of logging in the alt, and having to spend the coin to mail back to yourself, simply delete the alt.
4 Everything that was in his mailbox will be sent back to the person (your main) who mailed it, free of charge.
5 Recreate alt. Rinse/repeat.