Did you know you are making them break their license agreement?
The only legal licensing for a dedicated server in a datacenter per Microsoft is SPLA licensing.
You maybe are not aware of it, neither your customers, but if Microsoft decides to make an inspection in the DC your customers will be fined and will be in deep troubles unless they are using SPLA licenses.
You cannot buy a boxed Windows license and put in a server on a datacenter because if this are servers in a datacenter they are providing services to someone and this must be licensed under the Service Provider Agreement, the retail licenses are for internal company use or personal use. Not for hosting services on them.
It doesn't matter what other people tell you, look up here in WHT, it was said many times, the only and only legal way is the SPLA licenses. Any other type of licenses for Microsoft products is not. If that was the case, every host would just buy all the licenses instead of having a recurring costs, because thats how SPLA works, you pay each month.
I would wish you could just buy the licenses out, but you can´t. You need to lease them from Microsoft with this agreement.
Use the search button for SPLA on WHT and you will find hundreds of threads discussing every possible question you have about this.