Another vote for How Nice You Are 
I’d expect most people to put their mail on hold for a longer sit (or at least, if there wasn’t a fee as there is in Canada). The fact that you’re thinking of sitters wanting to order stuff is so nice!
Personally I wouldn’t mind picking up mail for owners for however long, and agree it would be part of the sit. I’ve had owners put it on hold and the PO didn’t hold it!; tell me not to bother with it; or, ask me to pick it up and let them know when something arrived.
I thought about ordering something from Amazon that I couldn’t get in Canada, back when I had a U.S. sit, but didn’t because I didn’t know if it would arrive in time (found it in a nearby store anyway, so yay!).
It sounds like you have an amiable agreement with the sitters either way.
As a pet AND house sitter, I would consider something so simple as part of the housesitting job. I live and pet/house sit in Europe and have accepted packages at the door (even if HO did forget about informing me). I prefer to know in advance, and the process (front door or back door, ring the bel or leave it), but can work without it. This last sit there was a very small package by the regular mailman that required a (small) payment of taxes. It is all part of my service….
@Felinelover, seems a very thoughtful question. We’d suggest openness in listing and video call - open, honest discussion goes a long way.
We’ve completed quite a few housesits that exceed one month duration. Several have included reasonable mail collection. No problem at all. Heck, one fabulous Pet Parent invited us to use their fancy side-by-side to access short-cut trails on private resort to reach mailbox - fun 
We try hard to be reasonable and thoughtful, and expect similar, in efforts to agree a fair trade. If you self-assess related expectations to be onerous on housesitter then consider leave a modest token of appreciation.
We have always considered getting the mail just to be a part of the sit. If you add it to your listing there would be no need to ask about it because sitters know when they apply that it is one of your expectations. It is lovely that you allow sitters to receive packages at your home and have even considered asking the question. Some homeowners do not feel comfortable with it, but I haven’t really ever understood why. The longer we are away from home, the more likely we will need a delivery.
Our sits are longer, like three to four months. For the past ten years of doing this, we’ve always had our mail forwarded, and if the house sitter wants to get a package delivered, our neighbors let them use their address. Also they can get mail delivered to General Delivery at the nearby post office. This year our sitter felt nervous about getting her packages and mail, so she asked if we could just not get our mail forwarded to our address at our other house. I figured it would be okay, but it turned out that we got a whole lot more mail than I realized. Our house sitter had to take pictures of all of the mail that she was picking up and then we had to ask her to open some because we wanted to see what was inside. In a couple of cases she actually had to repackage some letters and send them to us. I think she learned her lesson!