Word of warning - you'll notice how play-asia price most new games at $49.90 - just 10c under the threshold to encourage you to use a voucher so that you buy something else? Don't get tempted. Every time I have done this and ordered more than 1 item, you are automatically upgraded to the next level of packaging up (small box). In my experience and to my cost these always get hit by customs charges and that $5 you saved immediately becomes £8 Royal Mail admin fees + duty. Every single time (3) I've ordered more than 1 item even a game + a small packet of sweets for 25c to get over the $50 mark always results in a customs charge. I now only ever place orders for single items that way they come in a jiffy (bag that is

) and never get stung. I think this is due to the larger size of packgaing coming through a different shipping route. Regardless of how small the second item is, they always pack 2 items in a box and not a jiffy.
It sounds as though you've either been unlucky or have had at least one large item in your order. Playasia are the main company I use for my gaming purchases(especially since moving out here where alot of importers do not deliver to). I nearly always order a second small item to bring the value just over the $50.00 coupon requirement(usually tempo tissues) and my orders always arrive in the standard jiffy bag.
The order I placed today was for the 360 messenger kit along with Endless Ocean for the Wii, was still able to choose the cheapest postage option(jiffy bag) and I'm pretty sure that's what it will be sent in. I've only received one item in a box and that was because I decided to opt for it due to having better protection(was for my Logitech Harmony remote).
Was just looking at Scene-It for the 360 but the box for the game is bigger because of the buzzers. It's also £20 so does this mean its really likely to get clobbered for customs?
Not sure what the postage price is on that item, saw it there myself but the postage price wasn't listed in the item description, I think it must be a pretty large package so doubt the jiffy bag option will be there. I ask for all my items to be marked at $19.90 or less which they always do for me, works for me and they're very strict on import tax here in Argentina.
Of course there is always a risk of import tax when importing over the value of £18.00, but I think it's safe to say the majority of people who order from playasia living in the UK receive their packages without getting hit by import tax(provided they asked for the value to be marked down of course).