It may be more than you want to take on, if you need step by step instructions,.. but here goes...
1. loosen rear wheel
2. jack up rear wheel off the ground, (support it)
3. disconnect rear brake actuator rod
4. loosen wheel adjusters
5. remove axle bolt and pull axle shaft
6. remove drive belt
7. remove wheel
8. open brake drum
9. check to see if the shop really put on new brake shoes...
10. remove shoes and scuff them, and the drum with sandpaper
11. put everything back in reverse order
12. adjust belt tension, and align wheel
13. lower bike
14. tighten the crap out of the rear axle
15. adjust brake.
basically....
Others will tell me what I forgot.
As an alternative option, try doing a few hard stops in an empty parking lot and see if that helps brake them in...
The majority of your braking power comes from the front brake... My bike has 20,000 miles and the rear brake shoes are original and still in good shape. I'm on my 4th set of front pads....
Best of luck,
If you decide to tackle it and get stuck, feel free to ask us here.
ride safe