Why not? It's only money to you, if they cannot be your rivals, reduce your own business, waste your time nor cannot create an illusion you are not the original creator.
Nothing prevents them to use some other design if they cannot use yours. If your products have some original technical functionality (other than graphical looks) you can protect it by patenting. Nonfunctional creations such as layout, texts and graphics can be copyrighted. Actually the copyright is yours even without registering, but registering can give some evidence if that is needed. Patenting of functional principles is possible only if the thing is still unpublished. If it has been for sale, it's "already known".
You sell quite low cost items. Collecting royalties needs some system which is a pain. So, one time charging is the practical way. It can be "per design" ie. you agree which are the modified designs which they can keep visible and sell.
Have a written and signed agreement in two phases:
Agreement 1 which states
- they want to make new versions of your designs and you negotiate with them if in the future they can keep some of those versions for sale. They cannot keep visible nor sell anything which contains your designs before you have a separate written agreement of it
- they get some your material for redesigning your products.
- they cannot publish, give, sell nor in other way make available to others any material that you have showed to them or given to them for creating new designs
- either of you can stop further negotiations in any time and it's possible that you will never accept any of their designs.
Agreement 2 which states
- the exact design or several of them that they can keep visible and sell
- they have no right to alter that design without your written permission
- you are the main designer of the new version, the new version is a minor derivative of your design(image)
- what files you give to them and in what format to create a product version which can be sold in webshop.
- they cannot resell, hire nor give the rights nor data they get to anybody
- they must deny reselling, include some buyer specific watermark like item (you define it, it's a part of the design) use only a specified (non-easy to reuse) file format
- you have still exclusive copyright of every item you gave to them
- you have no responsibility to create nor accept updates nor give any support
- the name of the design that is used in their marketing
- how they present you as the original designer in their marketing
- the price they must pay to you
- the right to keep the new design visible and for sale starts when the price is totally paid
- when the right terminate
- how the agreement can be terminated before the written automatic termination day and what the participants get or must pay
- what is the court of law in case of conflicts
Termination day = the day when they no more cannot sell nor keep visible anything you have made; this is because you may want to use the stuff in a new way which doesn't stand their business and you obviously can accept lower price when you know how long your stuff will be used.
Give only difficult to modify files, they should be able to insert easily only their own items, not easily edit the core of your creations. Be sure the new design presents you acceptably as the original designer.
You can hide copyright notices to some textures as small enough vector elements or also in bitmaps if you use invisible watermarking methods. You cannot hide them behind other objects because covered parts easily get removed in making PDFs
Quite a long list. The price = try to guess how much they could get money and you must get a substantial part, say 20%, of it if your creations are substantial part of their products.