Discounts to Service Customers

I was curious how many Parts Departments out there give discounts a "trade" or "wholesale" customer when their vehicle is in the shop. We have a few customers that we give trade price to over the counter and have been quoting and charging them full retail when they have us perform the work on their vehicles.
It hasn't presented an issue thus far, however, I want to be prepared if it does arise so I can respond with "I'm sorry, we don't discount parts put on in our shop" or a similar response.
Is this common practice or do you guys honor their wholesale/trade prices even when the vehicle is being serviced in your shop?
It hasn't presented an issue thus far, however, I want to be prepared if it does arise so I can respond with "I'm sorry, we don't discount parts put on in our shop" or a similar response.
Is this common practice or do you guys honor their wholesale/trade prices even when the vehicle is being serviced in your shop?