Otherwise, do you have suggestions of what I could do?
Have you tried shooting them a note to see what they have to say?
I'm with you: a brand new keycap should not be shiny after a mere few hours of typing. However, because this is Unicomp, I find myself wondering if this might be another manifestation of their QC problems (which they seem to have had a fair bit of in the past). On the other hand, it could be strictly a one-off and contacting them may result in their coughing up a replacement keycap.
By contacting them, you will give them a chance to put things right. Likewise, if they
don't respond, this will allow you to make a better informed decision as to whether or not you want to buy any more of their gear.
However they respond, please do post their reply - it may prove instructive to anyone contemplating a purchase from them.