Why can't a medical supply company tell me what my copay will be before I order stuff?

I'm new to CPAP. I'm been using it for one month now and have found it to be challenging. I started off with a F30i that I had a hard time tolerating. I switched to a P30i and have been doing much better with that though my insomnia doesn't seem to have improved.

The one month mark is the point where you can start ordering new supplies. Quite frankly I don't feel that I need new supplies and I've only been using the P30i for a couple of weeks. However, the medical supply company has been calling daily to try to get me to order. Today I finally called them back. I asked them what the copay would be on my supplies and the sales person told be 20%. I asked 20% of what? She couldn't answer and said I would have to get in touch with medicare. We all know how difficult it is to get in touch with any government agency! I declined to order supplies at this point and asked her to have her company (Lincacre) stop calling me.

This is one of my pet peeves with medical practices in general. Why do costs have to be a mystery until you get the actual bill? These are supplies. The supply company is billing my insurance. They should be able to tell me the copay at the point of sale! Imagine if other companies operated this way? Would it be OK for a car salesperson to say, just take the car home and we'l send you a bill in a couple of months?

Just venting really, I know I'm not going to change a broken system. The idea that CPAP supplies need to be replaced so often is a scam anyway.