They are the same size.  The only difference is they have express post on it.  They have the same codes as the 500gm & 3kg c&s regular satchels.  

I sent some c&s satchels today and used my regular ones as I could not see anything about the express ones.

i believe they will be probably be the same price to post as the codes are the same.  The difference will probably be the price you buy the satchels for on e-bay.  Where as you pay around 15cents for the regular ones you may pay a couple of $ for these each which would cover the express side of things.

just what I think anyway.