I had a service visit booked at Winchester in May and asked if green plates could be fitted during the service as an extra.
I was quoted the same "under £30".
The service required was not completed that day, in spite of me waiting in the reception for 3 hours, so it needed another service booking.
However they had replaced the plates and left the old ones in the 'sub-trunk'.
When the second booking service was carried out, this time with a loan car included, it was zero billed as being under warranty and I never got the bill for the plates and no mention of them??