Buy the extender!
And if you ever *gasp* plan on selling your pochette you could always just use your extender as a keychain! I have one and it gives you a lot more room so you can definitely wear the pochette on your shoulder, no problem!
I bought a used extender on eBay for about $75, it was a little expensive for its condition (lots of scratches) but it would've gotten a used look from me anyway! Search "pochette extender" or "vuitton keyring" or similar things...and you should be able to find one. Sometimes there are some for very, very low starting bids; you just have to browse!
Congrats on your black MC pochette!