Firstly 5-6% is supermarket-chain margins. I'm struggling to understand how any small business could survive on margins like that, because the volume needed to make enough money to cover even minimal overheads is vast.
And volume helps with Paypal too. Here are their merchant rates according to monthly sales volume:
£0.00 GBP - £1,500.00 GBP3.4% + £0.20 GBP
£1,500.01 GBP - £6,000.00 GBP2.9% + £0.20 GBP
£6,000.01 GBP - £15,000.00 GBP2.4% + £0.20 GBP
£15,000.01 GBP - £55,000.00 GBP1.9% + £0.20 GBP
above £55,000.00 GBP* 1.4% + £0.20 GBP
As you can see, if you're doing the kind of volume that makes 5-6% margin a viable proposition you'll be paying 1.4%, not 3.4%. Still higher than with other merchants.
But I suspect that another problem with such a low margin is chargebacks and reversals; regardless of the merchant service you go with they'll be crippling.