I've never gone overland to CAmbodia, only byplane. In any case while I understand the principle of not wanting to overpay for a visa, the prospect of not having to spend an extra day or two dealing/going to/fr an Embassy and dealing with the lines, bureaucracy etc. would be far outweighed by a possible $5 overpay at the border.

Actually the China limit on US citizens for the last two years has been 6 month visas, not 90 days, again its best to do in Hong Kong (or Macao) rather than anywhere else as the documentation required is less, and the turn around time much faster. Indonesia should ALWAYS be done on arrival, unless you need a longer duration as the VOA is valid for a month.