For that classic finishing touch, I offer 100% modern mailable wax seals ("mailable" means that it can be handled by the postal service's machines and still be delivered intact... "traditional" sealing wax is brittle). There are many color options available, so coordination to your color scheme is a breeze!
Outside of the cost of the stamp, pricing for wax sealing is:
$1.00 per envelope for 1-20 envelopes
$0.50 per envelope for jobs over 20 envelopes