Maybe i havnt read this correctly, but surely it is easier to set your stall out as follows,

Sell your products to Everyone (public and whoever) at one price (set by what ever variables you work to, overheads etc etc) which only gets cheaper when higher quantites are ordered, so this elimiates your problem, tuners can buy single units at a price they already do, GP can if they wish buy at the same (you state your mission statement "we sell to everyone" clearly so there is no misunderstanding) and if tuners or dealers what a cheaper price then can if they buy in mhigher quantities