It is a real problem if you don't have reliably reliable transportation. Can you bike to the location? Catch a bus? Borrow a car? A scheduler will generally forgive the occasional need to reschedule because 'life happens', but unless you have done a number of shops for this scheduler already with no issues, two reschedules in a fairly short period of time may get you on her 'trouble shopper' list. If you absolutely have no way to get the job done, the best you can do is apologize and hope for the best. Hopefully this is an August shop with enough leeway to get it done without interfering with other August shops.
Is there a cab company, Uber, Lyft? A friend who will ask no questions if you tell them no lies? Yes, it takes out of your pay but in the long run, might be better than getting the shop canceled and upsetting the scheduler. I had the same issue last year, but I was lucky enough to be doing work for Strategic Reflections and they were totally understanding. They allowed me to do jobs originally due on the 7th to be completed when the car worked through the entire month. In part because I WAS RELIGIOUS in communicating with them and ALWAYS got the shops done.
I understand completely. Just stay in constant communication with your scheduler!
My alternator AND battery both needed replacing two weeks ago -- got it back, shopped one day, and took it straight back to the mechanic. He thinks now the fuel pump needs replacing, as the car is hesitating on acceleration. It was in the shop 4 of my 5 most important shopping days of the month! I, too, live in the boonies with no public transportation; and my nearest shop is 45 miles away!
Just communicate, communicate, communicate! Also, apologize, and own it.