I am not sure about the clear envelopes but I didn't know that if you have a square envelope and a rigid object there is only one surcharge. So- if you do have to pay for the surcharge on the clear envelopes go ahead, make a square and add all the rigid objects that you'd like to add. Usps.com has the guidelines listed there. Just go to calculate postage. Also I agree with another poster who said you will be upcharged more at the post office. They also told me they won't hand stamp anymore on request. So I get around that by paying the surcharge and making squares.