FR: Allow limiting maximum value of markup rate

[sparkalow]

What are you trying to do?
It's great that we can set a flat or percentage value to markup provider rates. It would also be great if we could cap the markup rate to a max value.

What's your proposed solution?
Probably a new settings field for max value. An event could also work but may be harder to implement.

Additional context
This came about due to a client having some products that are drop shipped or shipped from different shipping origins. To cover these 'unknown' shipping costs they are relying on a markup rate. However, they don't want to lose sales due to seemingly inflated shipping costs and limiting the markup rate amount could help in this regard.