Disclaimer: I have never dealt with DP. My comments are about small vendors in general.

I think you are missing the point. Many vendors get 100's of emails a day. After a while, they may start to ignore them. Not that they should, but many do.

The reason is simple. Often because emails are inquiries that do not lead to sales. It takes a lot of time to type in responses to email inquiries, too.

The solution is very easy. Pick up the phone and call them. Vendors know that anyone willing to pick up the phone and spend a few cents, either has serious questions or is ready to order.

You complained about poor service. I do not disagree with what you have stated. But you will note, that when you called, you got an immediate response. When you emailed, you got not much.

It has become the case for many businessmen that email is very voluminous, and not very profitable. The vendors are simply paying attention to those customers more likely to purchase something.