Buying Mac not at Apple store How much for Apple store charge to transfer data?

You can do it yourself with migration assistant. I strongly advise you make a TimeMachine backup to an external HDD first regardless of who does it.

Attach the backup to your new Mac and run MA. The process is about as easy as it can get and this will be an excellent opportunity to get to know your Mac better all around.

Move your content to a new Mac - Apple Support

You're not talking to Apple here. Ask the store that you're going to buy it from whether they would perform the service and if there would be a charge for it.