Work at the head of your flow at a 45 degree angle and as you stack packages for optimal carries, test packages for weight and shifting contents. It doesn't take much - but a brush of your hand. Then you'll know whether you can put 2 envelopes and a small or whatever and successfully carry it into the car. Don't build carries on stacks. Don't build multi car stacks or grab multiple items thinking you will remember. As much as you think you can, or you do, it increases margin of error.
if you're overloaded it's pointless to stack 5-8000 HINs. You can usually quickly reach into a car and hit the smart scan beacon and step out in just a few seconds. Never stack RDR/L either. Running a small to your last car while the flow is light might seem pointless but if your next boxed are all for your first car, it won't mess you up.
This helps me and might not help you but hopefully it makes you reevaluate your loads to find a better way to do it. I like stacks 3 or 4 high that I can grab with a hand and pull tight to my chest so I can write HINs as I walk and still use peripherals to scan for labels or bands or other
to trip on.
Good luck. Be lucky you get 15 seconds. They want us to do 1 in every 5-7 seconds or three in 10 seconds or less, LOL