If you're using USPS branded Priority Mail boxes, skip the measurements. They're not needed. Use them only if you're shipping a regular box that doesn't have Priority Mail printed on it. I think the only ones that will cost you more are the ones over 1 cubic foot going to zones 5, 6, 7, and 8 (see lurch-deeann's post above).
Don't worry about the fact that buyers purchasing multiple items don't have the correct postage before they bid. They know that it will either be the same (if sending in separate boxes, which is unlikelu, or go down, if combining shipping to one box). When you send them the invoice for multiple items, you can figure the measurements then. Use USPS.com to calculate postage at that point.
Hope this helps!
2purrcats
