Personally, I wouldn't because it's too specific to the particular goods being sold and thus can't easily be abstracted. Coffee is going to need to be handled v differently to candy or women's shoes.

I would offer a service up to the point of providing the labels and packing slips to the merchant and let them execute the fulfillment themselves.

My guess is that for most 'normal' people the website, payments and subscriber management aspect is the most difficult part of the operation to run so it seems best to focus there.