I've had half-frame done by mini-labs OK. Order 5x7 prints and you'll get two postcard-sized prints from each. Whether edge cropping is acceptable will depend on the lab and on your own tolerance.
It really isn't difficult -- one might even call it a no-brainer -- but unfortunately there are always a few minilab operators who do, indeed, have no brains. If anyone is bright enough to understand what you are talking about when you take the film in -- as most should be -- then they should be able to process and even print the pics OK.
Cheers,
R.