Does anyone have any experience with how well glassine envelopes go through the mail? My save the date's are going to be postcards with small coin envelopes attached to them with a string of numbers that make our wedding date inside. The problem is, I can only find regular paper envelopes in quantites of 500 for $30 or more, and I only need 75 of them or so. I can find glassine envelopes for $6 in a much more reasonable quantity, but I am afraid it would tear open and spill out my numbers. Thoughts??