The package is borderline between "small" and "medium"
And between the "up to 500g" and "up to 1kg"
So it will depend on which of the post office counter staff take it in
If they consider it small, and under 500g it would be £3.00 postage plus the baggy price.
At the other end of the scale if they consider it medium and over 500g it would be £5.10 plus the baggy.
A couple of grams or a couple of centimeters will tip it either way, sorry.
These are taken from the Royal Mail page "Get A Price" - i hope they're feeling nice in the P.O. this morning.
Leila.
p.s. Lollipop - I have PM-ed you about the name you'd like on the package?