I’ve had another thought about this though, including the P&P in the price takes away the incentive for people to buy several items. Normally when I buy beads there’s a main postage price then a much smaller additional postage cost for another item. I recently bought several lots of beads from a shop on here which only charged about 10p postage for additional items. I ended up buying nine or so items I think! 
See this thread for how this was done: