That is correct, Rules Engine can be used.
Make sure to add all services that you might need to your Royal Mail integration first. Once that's done, you can create a few rules to check service/weight/dimensions and assign appropriate services.
Please refer to my earlier reply for a rule sample:
Is there a guide on this?
Forgive me if I missed something, I'm new round here... please go easy on me :-)
I have items listed on eBay which are under 2kg, therefore Royal Mail 48 is my preferred carrier for these.
However some of these items are letter size, some large letter size, some parcel size.
When mapping the channel shipping options in the Channel Integration, I can only specify that Royal Mail 48 on eBay is mapped to one of the OBA Mail Services (I chose CRL Royal Mail 48 Parcels) however dependent on the size of the item / order, this may not be the most cost effective to use - it could for example go as an STL letter which is way cheaper.
So am I right in thinking that I could use the Rules Engine to assign the correct Shipping Type (letter, large letter, parcel) based on the weight and dimensions of the package?
Confused of Yorkshire.