A few tips to follow after you order your invitations and before you mail them out.
SEALING ENVELOPES
I always use a Tape Runner with Adhesive like this one:
http://www.amazon.com/Xyron-3301-12-32-Runner-Forty-Adhesive/dp/B004QM94KU
For all of my own envelopes.
You never want to use just a seal because the corners of an envelope if not sealed can get damaged.
Use the adhesive from the gumming edge to edge then press for a seal envelope shut.
It works great! No licking tons of envelopes or over wetting edges with a sponge.
POSTAGE
Take one envelope stuffed exactly how you will mail it to your local postmaster for current postage rate.
Then you should buy your stamps and apply them.
REPLY ENVELOPE
Add a postage stamp to each envelope to make it easier for your guests to respond.