Late on a shed payment

You need to hire an attorney right away. And callthem to bring the shed back. This is a huge mess.
It would be good if, when the shed comes back , you have an independent witness there, try to make the shed people stay there while you open the shed and check the belongings. The problem is , the contract you signed when you bought must have somne "self help"language in it that you agreed to , and if you were in default, they had a right to come get it ( if the contract says so) and there is probably language that state sthey are not repsonsible for consequential damages. That is why you need an attorney, to tell if the contract is valid, and or you can overcome the language under some legal theory.
Your state law might have required them to cut the lock and remoive the items before repo-ing, but only an atty in your jurisdiction can make that determination. I would think they still would be required to take due care, and not act spitefully, capriciously, or recklessly with the property of another.

Geez,. what if had rabbits or a dog in there.

Agree with the above, but I would start immediately making a list of things that you had in there as well, just to make sure you get all your things returned.

Also, you mailed it certified, did it arrive before they repo'd the shed? If so, they may be in breach of contract if you had a prior payment agreement.

Start documenting all of this today with as many names dates times and places so that when you speak with an attorney, you have most of the information they'll need.

Secrect agent, is right. I had not thought about them , then cutting the lock off and absconding with the things.. Though we have all heard about that happening with stuff peopel have inside their cars when they are repo'ed or towed.
These new "purchase" contracts are becoming more and more like "adhension" type contracts; in that there terms are so favorable to the "lender /seller" that the buyer/ lendee, sign away so many of their rights.
I can just imagine the damage to all of that stuff as that shed was cranked up on the flatbed.
I have seen some where that even when you pay, the "company" STILL repo's, using their contract , which says that they can still repo, even when paid, because once you are in breach, the breach cannot be cured. so you pay them and still they repo. Unless, they get taken to court, and have the money to hire an attorney to hold their feet to the fire, and find some statute or public policy violation the at invalidates some or all of the contract, they get away with it.
The "poor get poorer" due to the lack of regulation to make contracts fair for both sides.