Well be careful he doesnt just take off the with xbox or claim he never got it in the mail.
Also be careful that he doesnt swap your dvd drive out for a worse one or any parts inside your xbox.
Iv heard storys of people sending in felcon boxs then getting back DIY fixed RROD Zephyr boxs.
I recently upgraded the firmware on a so called jasper box which turned out to be a felcon with a memory stick soldered inside of it to make it look like it was a jasper.
The guy was pritty sure he had a jasper because he had the correct case for it and the correct box and had brought it new from DSE. But had done a mail in mod job the week after he got it so I recon that guy that modded it swooped it out with his diy soldered memory stick one. And was told he had to use a new psu that got sent back with the xbox after modding because the mod used more power.
IMO its better to do it your self or get it done locally where you can watch.
Its not really that hard to do xboxs.
1 out of 10 it would be a 2.5 for sam sung and benQ drives, 4 for hitachi drives and liteon that drive would be a 6 for a first time flasher.
No soldering is required with xbox 360, just to be careful and fully read the instructions for flashing your drive type.