It depends upon the client and the amount of work they provide, how long you've worked with them, how imperative they are to your revenue, etc.
In most instances, you should absolutely charge more. Without question. However, how you charge can go several ways....
Ideally, you'd configure payment based upon usage. If they are a trustworthy client you've worked with for some time, this is a great option. They pay the standard rate for the design, then $X every time they reuse the template. This does depend upon the client being ethical and honest. In a good standing relationship that shouldn't be a problem. For a new relationship, you may not know.
The second option would be a larger base fee and then no per usage fee. Charge something such as 5x or 10x the normal amount. If they are reselling the design they should have no issue recouping that expense on their part. This compensates you for reuse while eliminating any need to track actual usage and to trust the client.
A caveat I may consider would be for a great, long-term, client with which I really enjoy working. And how often such requests for a "template" reuse are made. If this is the first such request in a long business relationship after many projects have been completed and paid for, I may just let the client have one such "template" for the standard rate - more as a "bonus" or "thank you" for their patronage. I'd explain that it's a special term for this piece and in the future we'll need to discuss things more (see above). It's not unheard of to let some things slide as a "loss leader" to maintain great a business relationship. If it's rare that they make such a request, then letting one go here or there isn't the end of the world.
But make no mistake, the default position should be to charge more.