For mail forwarding we use Flat Rate Priority Mail. This works great as long as you have someone picking up your mail at your home.
Before we leave on an extended trip we call the post office in that location to confirm we can have mail forwarded to us c/o General Delivery. If they will, we pre-address the Flat Rate Priority Mail envelope to that location and affix the appropriate Flat rate postage. We ask the Post Office how long the priority mail will take from our home zip to the zip where we are going. Usually 3 business days, but if going to a rural area it can take longer. Then we leave instructions with the person picking up our mail on what date to send our mail to us c/o General Delivery. The only drawback is the Priority Flat Rate Mail Envelope must be taken to the Post Office Counter. It cannot just be put in the outgoing mailbox.
Your mail forwarder can stuff whatever mail the flat rate envelope will hold for the one price. We have used this for years for trips up to 3 months and only once did our mail not get to us before our expected departure from one RV Park to another and this was our fault since we forgot to find out how long it would take from one zip code to another.
Safe and happy travels to all.