they can give it to you at their cost, and prove it with invoices.
If their cost is higher than you can buy it from LP, then buy it from LP instead. The shop should not care (unless they're lying with a fake invoice) since there's no profit to them in either event, and thus no profit lost.
I have no problem "advertising" something for a company I work for. But, if its a condition of the job, I should get it at cost - there's no reason for them to give it to me, but there's also no reason for me to PAY THEM to work there!
BTW I have a Mk25/S600 and like it a great deal. No problems with it breathing wet or strange in odd positions; its only "quirk" is that it has a funky noise on inhalation when iinverted AND turned to one side (a very odd position that you won't often experience); there is no effect on effort or breathing though - just a kinda odd noise component. It may just be moisture inside rattling around on the poppet.