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. Patenting is the only way to prevent it, if you have something really original, which they actually must copy, too if they want to make business with your ideas. Patenting probably isn't possible any more because you have put your stuff 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 only practical way. Have a written and signed agreement which states:

- the price
- what files you give
- what they can insert and also what they cannot
- they cannot modify your designs, only insert the listed own items
- they cannot resell, hire nor give the rights they get to anybody
- they must deny reselling, include some buyer specific watermark like item if you use it and use only a specified (non-easy to reuse) file format
- you have still exclusive copyright of every item they get right to use
- you have no responsibility to create updates nor give any support
- how you are presented as an original creator
- when the usage 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 by others.

Do not give anything before you have got your money. 

Give only difficult to modify files, they should be able to insert easily only their own items, not easily edit your creations

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.