Rich
You are right, get them to pack it as small as possible as this saves on the cost of shipping. I always us USPS for shipping as it is tracked (6-10 day delivery). I think they collect from the seller also so they shouldn't have to drive to post it.
Remember that it may get caught up in UK Customs in which case you will incur VAT, customs charge and also Parcel Force charge, there isn't really a way to get around this, sometimes you get charged sometimes you don't it seems to be pot luck. You can ask the seller to declare the parcel contents at $0.00 but then if you have to claim if it goes missing you will get re-imbursed this amount.