After choosing the invitation, the next most important element is the envelope--the first thing your guests will see to set the tone for your event.  There are differing elements in envelopes:

Size
Color
Proportions
Paper quality and texture
Lining...if any.

Consider your mailing list when choosing; do you have many long names/addresses?  Have you chosen an elaborate, flourished script?  Will it fit nicely?  Allow for postage---will you have to affix several stamps?  The bottom 1/2 inch on the envelope will 'belong' to the post office for a bar code and usually the top 1/4 to 1/3 is left blank for 'framing' the address.   

Ink-worthiness is a major consideration.  Most likely, this is the first major event you are planning and there are myriad details to attend to...from catering to transportation to music.  Do not allow yourself to be forded to accept less than what you have envisioned for your invitations because the paper will not 'take' ink.  You may be budget-conscious but a poor choice in paperstock will prove more costly in the end as a calligrapher may charge extra for having to write on poor paper ( it may bleed, shred or be difficult to write on)...if the calligrapher can write on it at all.  TEST THE ENVELOPE PAPER WITH A FOUNTAIN PEN TO SEE IF IT CAN TAKE INK.

Flourished Copperplate Envelope and Escort Envelope

 Envelope done "Copperplate" and Escort Envelope done "Flourished Copperplate"

with Hand-painted rose accent