Kane, yes you are spot on with your understanding.
We pass other sites business, they pass us a cut of the sale, we hold those cuts until a set date then whatever the total is, its given as a form of credit note to one customer to be used as and when they please.
So say, it made a total of £1200 in the year then they have a credit with us of £1200, they can spend this as 20 x £60 half hour bookings, 11 x £110 hours, one massive 5 girl 2 hour blowout or any combination they want it would be upto them.
As for who wins, its just a basic thought at the minute so weve not fine tuned it but every booking is logged by us, we could just give one entry per booking or one per half hour (ie book an hour and you get 2 entries). At the end of the year they all go in a pot and 1 is drawn out. That part shouldnt be complicated (and to be honest I didnt think the 1st part would be).
Eddythedog: Its a thought, but one many agencies regularly do, we were looking for something bigger and more unique (plus this "potentially" big prize wouldnt actually cost us anything, in fact we'd still get our fee of the bookings the winner then made!

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