Sorry man, I'm way out west in UTM Zone 10 (BC).
Here's something I found if you want to do it yourself;
How To: Change the spring in your AEG (Airsoft) - YouTube I quickly checked out the video. It's not made by a 12 year old and the guy seems pretty competent so you should be fine following his instructions.
BTW the $30+tax doesn't include purchasing a spring (like $10-12) so I could see it being $40-45 if you need to buy an M90 while you're at it.
EDIT: Kokanee does bring up a good point about shops backing up their work though and I assume you'll be able to test your gun in the back to verify that it actually works and is the FPS you want it to be. And yeah a lot of competent gundocs are folding these days or not advertising their services anymore except to people they know (I think it's partly because of the flood of kids and "young adults" in my age group (18-25) who are whiners and part of the "I want it nao!" generation).