I don't have actual experience shipping DME to WDW, but your post brings several questions to mind that you might want to consider:
Shipping costs are largely determined by size and weight of the package to be shipped. Obviously, a wheelchair would cost more to ship than say a nebulizer, and a power w/c would cost more than a basic push w/c. You should be able to insure anything you ship, however if there is damage it likely would take time to make the claim and receive settlement; you would not get immediate replacement and you would need to make alternative arrangements. Definitely take pictures from all angles just prior to boxing whatever is being shipped to proof the condition of the item prior to shipping.
Lowest shipping costs are typically associated with the longest shipping times. Do you have "spare" equipment that you can do without at home both prior to and after your vacation?
Medical equipment can be taken on an airline free of charge and without counting towards your baggage count (either checked or carry-on). You specifically mention a wheelchair - how will the person get through or around the airport without it if you ship it ahead?
Again, considering the possiblity of damage or loss during shipping, it might be better (and possibly more cost-effective) to rent the item once you arrive in FL. If anything did happen in transit, you'd end up renting anyway plus you'd have the hassle of filing a claim, etc.
I think shipping is definitely POSSIBLE, whether it is the most convenient or economical is what you would need to decide. Good luck!