I received one bill, claiming past due and with a ...

I received one bill, claiming past due and with a threat to be sent to a collections agency. Only bill I've ever received from them. Seems they do that to a lot of people. Just read the reviews. They will continue to claim to you that another was sent prior. Relying on a "timestamp" from their thirdparty mail service as their "proof". Not a business I would recommend.
*Update* 9-28-2020
Not only did they not send the first bill (spare me the regurgitation of the "process" of mailing three out), but they overcharged me as well. It was NOT a mistake of my insurance, it was an "oversight" by National DME.
I suggest everyone ask their healthcare providers if they use NationalDME, and strongly encourage them to change providers. This company is not only a pain to deal with (csr's just repeat the same phrases without answering specific questions) but the company seems to have an awful lot of "mistakes" and "oversights".

*Update*
Since Google doesn't allow for a reply to reply, I'm updating here. Per the disingenuous reply from the "owner", simply stating that you received "notice and postage bill" from your third party mailing service is not proof of a statement being mailed, as you should know. Those notices and any billing you may receive from your third party is by no means proof of it going out, as your employees agreed during my phone calls. As far as your CSR's go, you can think of them as great, but I suggest you raise your standards. Because the two I had the displeasure of dealing with are incompetent at best. Regergitating the same platitudes and "policy notes" isn't service. Someone claiming to be a supervisor's supervisor was the only person to help, and getting to that person took forever. And that is the person that offered the discount which I never asked for nor was I seeking, but you already knew that. So I'm at a loss as how turing down a discount is "some customers don't want to be helped". Care to expound?
And good job on blaming the customer and not touching on the over billing (fraud much?). I'm sure that was just a "rare oversight" right?
Like I stated, people should demand to know if their healthcare team does business with NDME, and then insist on going elsewhere.