If it is sewing fine, I would give it a good clean out in the bobbin area and put a drop of oil on the hook and sew until she started acting up. If the technician doesn't know that brand, it could take quite a while to get it sewing well again. Unless they have loaner machines while yours is in the shop, I would wait.
I would also be a little suspicious if they called with a major expensive fix if it was sewing fine. Then what do you do? Okay it for them to continue or bring home your non functioning machine.....just my opinion.