Hi
From Betfair:
"What is the minimum stake for an SP bet?
The minimum stake for a back bet at BSP is £2 or the local currency equivalent as per normal. The minimum liability is £10 or the local currency equivalent."
Allow small bets is accomplished using three bets. This can be done at a specific price, but because the SP is not known until the bet is matched, small bets procedure will not work. Effectively if the SP bet is below the stated minimum it will be rejected by Betfair.