Different VAT on postage

[tjs-cloth-nappies]

Hi I am just building my website and having trouble with the VAT on postage

 

I sell nappies 0%, sanitary 5%, bags 20% so which ever the customer buying the postage should have the same VAT rate on.

 

ie they buy a bag 20% the postage should be 20%

ie they buy a nappy 0% and a sanitay 5% the postage should be the higher of the 2 so 5%

 

Dos any body no how to make this work please, If this will work.