I think it is a common problem. Around here the store employees won't help either. I'm thinking that maybe a "dog ramp" that is used to help dogs get out of the trunk plus a dolly might work. You'd need one that is rather smooth instead of having "bumps" that are used to make it more like tiny steps. I don't think you are alone is this problem. If I'm in a store and know someone who needs help, I go outside and help them. However, that can be a risk...like what if I drop the machine? or hurt my back? Those possibilities increase as I get older.
I have a small dolly that I place the machine on once I get it out of the car. Then I can just pull it into the store. I can understand why a business does not want its employees getting machines out of the car. The business would incur a liability if the employee gets hurt on the job and if the machine is damaged, they'd incur a cost for that too. Those costs can be extremely expensive for them.
A lot depends on the store employees. One place I take my machine has a guy that comes out and gets the machine. Those places who have machine repair service are more likely to do that. Do you have someone at home who can help get the machine in the car? Maybe they could go with you to help get it out. I'm 77 and work my arms with weights (not the big ones!) to strengthen my arm muscles. I've found this beneficial for me for around the house tasks too.
I look forward to see how others deal with this problem.