Example,

This, with packaging weighs 300g, and I can post it to you today for £2.20 second class post, realistically I'd charge £3 P+P
On Tuesday however, because the item is bigger than 8cm in height it's going to cost £5.20!
Nobody is going to pay £5.50 to £6 P+P on an item that's probably not even worth that!
So what are my options? For heavier items I was considering www.collectplus.co.uk, it would still make sending the air duct above possibly prohibitively expensive, but may work on heavier, and more valuable items.
Does anyone have any views, advice or opinions?
One thing I should add, I'm selling on eBay to fund my mini, selling less than 20 items a week, and I'm out at work during the day so I don't think a courier is going to work (although I do use them for some heavy items currently)