Once you have your confirmation number through www.uppersia.com, is it possible to just send everything off in the post with an SAE and get your passport sent back with your visa stamped in?
For visa you should fill in the visa form on Iran visa application , it takes about 3-7 working days to get your reference number and two working days to collect your visa at the embassy by direct referral or one working week by posting the docs to the embassy. You can pay the visa fee in cash in Iran.
After being stamped in your passport, this visa is valid for 3 months to enter Iran. The exit may happen after this period. You can collect your visa in any of the Iranian embassies all over the world which should be specified while sending your request.
Check the cheapest Iran tours.