Never had any problems with Rider Proxy, however a shitload of people were pre-ordering with her and paying 1000 yen and then douching out on it, leaving her with tons of stock of shit she didn't sell. That's probably why she doesn't allow for preorder payment plans unless you're a regular.
I've done a lot of business with her, and she asks me what I wanted to have for the initial deposit because she knows I'm good for it. (Paid the entire total of the figure because I like knowing I just have to wait for the shipping quote, much easier)
Also, you said she didn't respond until after you paid her? How much time did you give? Because maybe she was, I don't know, busy, and you were impatient?
Like Suri said, I've never had RP round up shipping either. She charged me for the same EMS price as what she sent it out as, if not /less/ than what she spent.