Maybe I'm a bit paranoid but when I looked at the advertisement for traveling mailbox I noticed that one of the users that praised this service had a Nigeria address( not that there's anything wrong with that). I also read a couple of suggestions by other posters in this thread that implied what they thought the post office did for somebody's brother-in-law. Having proven myself to be my own worst enemy I decided to call USPS and find out what they actually do rather than sign up to a service and find out that I wished I hadn't. USPS told me I could sign up with the USPS for informed delivery and they would send a scan of each piece mailed to my address on my phone.(or email I think but i didn't ask specifically) USPS told me I could have my mail forwarded to a different address and it would take about a week or so and about the same length to bring it back to my permanent address. They said the mail could also be held at the post office for 30 days only. The post office didn't say anything about a fee for informed delivery or mail forwarding so I don't think that they charge for that in fact I'm pretty sure that they don't, as the PO is usually pretty quick about letting you know about any costs up front. Travel mailbox will shred, sort and forward for a small fee or a large one depending upon what you have them do. I've got kids that could do that. I've even got an ex wife do that could do that and one that could not. Thinking to sign up for informed delivery to see how it works for a month or so for when I go on that three or six week vacation. I am not so sure I can handle robo mail any better than I can handle robo calls. And I just realized need to ask how to turn it off once turned on although that may be just a personal fault.
I have used USPS Informed delivery for years. It is a great security feature and has helped track down misdelivered mail. As mail goes through the sorting machines, it is scanned for address and postage, the USPS will send you copies of those scans if you enroll in Informed Delivery. However, it does not even come close to what a mail service like St. Brendan's Isle does.
Everyone should use Informed Delivery for security purposes. When I signed up for the service there was no real verification of my identity. Thus, I could have signed up for the service at random addresses and get notified fo the mail being delivered. When checks or credit cards come through, I could just intercept them. Only one email address per street address can enroll in informed delivery, thus, if you sign up no one else can.
With USPS you can not select which items to be delivered, which to be held, or which to be forwarded. It is an all or nothing deal, all mail gets held or all mail gets forwarded (except for junk mail). We use this when we travel or will be away from home for shorter periods of time. It is not suitable for the OP's purpose or anyone else cruising for extended periods of time.
St Brendan's Isle Mail Service has a long and respected history in the cruising community. It is what we use. In addition to scanning envelopes, they handle package delivery, opening and scanning documents, can assist with establishing residency in Florida. This kind of service is needed for the OPs intents.