Readable comes first
Decoration should never compete with the address. Leave clear space for the recipient, return address, postage, and any postal marks. If the envelope is heavily patterned, use a plain address label.
Flat and flexible wins
Many envelopes pass through machines. Raised buttons, thick charms, rigid trinkets, and hard corners can tear paper or require a different mailing class. When in doubt, choose a larger envelope or skip the item.
Clean contents
Loose confetti, glitter, powders, liquids, oils, food crumbs, and strong scents can cause problems. If you want a celebratory effect, print the confetti pattern on paper instead.