The Best Price Execution feature enables customers to get matched at the best available odds on the Exchange. With the Best Price Execution feature switched on, if better odds are available, or become available, after you have placed your bet request, your bet will automatically be matched at the improved odds.
The minimum deposit limit to add funds into Betfair is just 5, and this is the limit for most methods. The maximum deposit limit from the E-Wallets is 99,999. Debit cards may have a higher limit but this is likely to be set by yout own bank, not Betfair.