When sending christmas gifts I send the toys via Amazon.com so they are delivered usually free and go directly.
More personal stuff I post via Royal Mail. The 2kg limit is usually around £27. It's amazing if attention is paid to sourcing lightweight items just how much you can get in a box at under 2kg.
TIP: If possible wrap in bubble wrap and use a plastic bag (or a bin bag folded several times). You can save sometimes about 250g on the weight of a box.
Often I cut away any surplus cardboard not needed to enclose everything, to cut the weight, and then secure/reinforce it using parcel/masking tape.
If extra space is there I fill it with Tayto Cheese and Onion Crisps, made locally and highly sought after as a 'taste of home' 
Last week an Advent Calendar cost me £8 to post.