Royal Mail Size Problem

Hi Guys First of all sorry if this is answered else where in the forum(i did look).

I'm about ready to open my store (UK Based) and I would very much like R.Mail to be a delivery option however due to it working the cost out on a weight basis it works out the cost wrong.

For example one of the things we sell are posters, the postal tube has to be 62 x 5 x 5cm and 0.2Kg (including the poster)
the price O.C check out offers is £2.48 (1st class) & £2.16 (2nd class) however the actual price is £5.62 and £5.20.

Anybody know a way to resolve this?

Thank you in advance.