Can you send a bill payment without a windowed envelope?

Yes. Windowed envelope is just a curtesy.

It will get there a day slower since it won’t have the USPS barcode on the bottom.

Otherwise no big deal.

Sure.

I have a number of monthly bills I do this with, it’s fine.

Yes.

I do it all the time. I usually throw out the window envelope because it’s small and so to me, easily lost. I like using a regular 10 inch wide envelope (business size envelope). I just hand write the “to” and “from.”

Yes. Intermountain Gas Company does not give out envelopes any more.

Yes.

As long as they get your payment, that is all they care about. If they provide you a window envelope that’s for your convenience. If you are buying the window ones yourself, they are more expensive and not necessarily needed.