A few tips to follow after you order your invitations and before you mail them out.
I always use a Tape Runner with Adhesive like this one:
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.
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.
Add a postage stamp to each envelope to make it easier for your guests to respond.