Create your seals using Flexible Sealing Wax. It's formulated to be unbreakable and won't snap or crack like Traditional wax. It's ideal for items sent in the mail and creates an incredibly strong bond when applied as hot wax directly to your enevlope.
It's a rare occurrence but mail sorting machines can sometimes scuff a wax seal. If you want your item to arrive in picture prefect condition consider using an outer envelope. Tarditional wax seals are likely to be broken in transit, so always use an outer envelope for protection.
There's no substitute for a hand-stamped wax seal applied directly to your envelope. It creates the strongest bond. Self-Adhesive Wax Seals have a strong bond but tend to lift at the edge. While uncommon, they may be removed during sorting or transit.