Good.
And do you acknowledge that the most bulletproof way to prevent this potential 8x10 problem would be to actually crop the client's files to 8x10 for them?
Excellent. More progress.
And you understand the risk inherent in this? That the top of somebody's head could easily be chopped off, in a portrait photo for example? Or any other random limb chop that you yourself would NEVER allow in your own work?
Ok, good. That's some progress, at least.
And do you understand that your client, or their printer, are VERY unlikely to pause to consider your composition when cropping? They'll just use the exact centre section of the photo, yes?
That's right, they can print whatever size they want. Therein lies the risk in selling digital files. Depending on their chosen print size, there will be cropping. Are they qualified to crop YOUR photos to look their best? Of course not. No more than they are qualified to edit your photos. That's why YOU must do it.
Your photos are going to be cropped. That is the truth. It's much better that YOU crop them, don't leave it to your clients.
You sure as hell wouldn't give unedited photos to your clients and say "it's ok, they'll edit them for me, I'm sure they'll do a good job." Would you?
Millers is shit, that's an established fact. Yes, definitely get some test prints from other labs.
I don't know if I've already asked you this - are you certain the light in your room is bright enough? https://www.damiensymonds.net/2012/01/light-around-your-computer.html
You MUST read these two articles:
https://www.damiensymonds.net/2011/02/selling-digital-images.html
https://www.damiensymonds.net/2015/07/selling-digital-files-you-cant-limit.html
I will help you with your batching, but NOT until you assure me you understand why what you've been doing is wrong. NEVER give 8x12 @ 300 files.