You need to figure out what size(s) of envelope you will be using.
Will you be using standard #10 envelopes? Will you want a window?
Most papers have matching envelopes in the smaller sizes; some
have a window option, most don't.
If you need to use larger-sized envelopes, you must specify the
colour, size, appearance, and orientation of each envelope type.
Regardless of the envelope size, you must decide if you are going
to use return-address labels or pre-printed envelopes. You must
also determine how the envelopes will be addressed: printed white
or clear labels? Typed on directly? Hand-written?