I hear a footballer called Wayne Rooney once met a seller like this in Liverpool. The lesson learned then was that asking for tips never bodes well for anybody.
If you still have this order open, I would just ignore this, wait for your work and leave a review outlining your experience afterward.
It is true that sellers can take orders and realize that the amount of work is much more than covered by the price of the original order. However, in this case, a discussion with the buyer and the ordering of an extra to cover the shortfall will usually be arranged.
Asking for a tip is simply rude and especially in this manner, highly unprofessional.
That said, is it possible that your seller missed something like a smiley face off the end of their message and that it might have been a joke? - Just asking.