The answer is to pass the cost of consumables on to your customer.
I charge my customers £1 per tab, so a typical dining room set works out at an extra charge of...
6 x chairs x 4 legs = 36 legs
Plus a couple of coffee tables = 8 legs
Total legs: 44 @ £1 ea = £44
And as for the dining table, well I'd have to use those REALLY EXPENSIVE polystyrene blocks, which bumps the price up considerably.
Simples
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On a more serious note: When I put an order into a supplier, it's not worth trying to save £4 by sourcing one item from another supplier due to additional postage costs, and trying to make the order up to the discount point (typically 10% for orders over £100).