If you're only used to shipping with couriers that ship everything in a car or a van of some description, you might grossly overestimate the amount of weight that a bike messenger can safely carry. With your average Florida delivery service it is safe to assume that they are going to have vehicles of all types in their fleet, and there might be an occasion where it might work to your advantage to use one of their bike messengers instead of one of their larger vehicles. The principal reason for this is that shipping anything via a bike messenger is definitely going to save you some money if you're shipping small products in the downtown core of a city rather than shipping them with the larger vehicles the courier has available.
If you are having a hard time deciding whether or not your package might be best carried by someone on a bicycle, its best to just apply some common sense. Is this package something that you would want to carry on a bike? Safety is the primary concern for bike messengers, as they obviously have none of the protection that someone in a car has, so anything that is either to heavy, or too large, would encumber them too much and make them dangerous on the road. So, if you're shipping something in an envelope, or some other package of documents, you're almost always going to be safe. For physical goods, its best to stick to packages under ten pounds, otherwise the bike messenger might actually just reject your package.
The most important thing to remember is that safety is the consideration, not exact numbers. Just because a package is under ten pounds doesn't mean that its going to be especially safe to carry on a bicycle if its extremely oversized, so keep this sort of thing in mind as well.
Don't be afraid to just be honest with your delivery company when calling them for a messenger about the nature of your package, as this is going to get you the best shipping arrangement. If you try and save money by ordering a bike messenger when your package is actually to large for them to carry, they will show up and refuse to carry it, costing you a fee for their arrival as well as having to pay again for a larger courier to move the goods. Just asking before hand can alleviate these kinds of problems.