$5 buys you Raspberry Pi Zero: 512MB RAM, 1GHz CPU, MicroSD slot, 40 I/O pins, mini-HDMI, microUSB.
There's no way to save on shipping by buying multiple units, as the limit is 1 per customer.
I'm guessing shipping (the padded bag plus postage) costs them about 80p, so in reality I paid about $7, not $5. About $1 of that is tax.