Airport Express/AE Travel Pass with two trips (for 5 days)

I am going to be in Hong Kong from Sunday night to Friday afternoon. Should we buy an AE Travel Pass with two trips to/from the airport? How much does it cost altogether? I am not sure what is meant by 'The price is HK$220 and HK$300 respectively (with HK$50 refundable deposit).' And can we get a full refund of the remaining balance at the airport before we leave? For the two days that are not covered by the three-day travel pass, should we pay for each trip or buy a day pass?