If the straps are indeed different lenghts, 2 options,
1) get matching straps, shouldn't be dealing with that sort of thing on a new kite, The store you got it from should have no problem sorting you out.
2) instead of shortening lines, get a little bit of bridal material, make sure it's rated at least the same as your flying lines and make up an
extension for the shorter one. You don't want to be tying knots in your flying lines to shorten them. It'll weaken the knots, What I do in a pinch is
make up an extension, it has a loop on one end to larks head to the handle, then a knot on the other to larks head the flying lines on.
I usually have a couple of these made up in the kitebag, I keep them about 6-8" long, with a knot every 2" or so. They're great for tinkering with
kites when you get one that the brake lines are too tight. Just make sure the knots on both sides are the same distance from the larks head end.
Sysmic S1 Buggy.
0.7m / 1.4m / 2.0m PKD Buster I
4.4m PKD Buster
10m JoJo RM+
6m Flysurfer Outlaw
12m Ozone Access