Just got a lesson from my father-in-law about giving money gifts at Japanese weddings, so I thought I'd share the info:
- There are several types of envelopes you can buy. Make sure you don't get the ones without any gold or red on them (those are for sad occasions) and make sure the ribbon is real, not artwork (the artwork is for tips that you give people in day-to-day activities, but not weddings)
- The cash part of the envelope is inside, make sure you put your name and address on the front of the second envelope, and the amount given on the back
- The range of values vary depending on where the wedding is and your social station, but the minimum is usually 30,000JPY per person. Also, if you are combining cash (if a couple is attending), make sure you give an odd number of bills. Even bills are considered bad form, because you can divide them between the bride and groom. Also, don't mix bills: give all 10,000JPY notes
I'll ask for some more clarifications tomorrow, but hopefully this will be helpful to some of you.