While these instructions are correct for NCV3s, I believe they apply to your customer's T1N Sprinter.
https://sprinter-source.com/forums/showthread.php?t=4688
The dipstick should "bottom out" with more than a couple of inches hanging out the top of the tube.
Confirm your dipstick looks similar to the one I've pictured. While there are several types that work with the transmissions of older and newer North American Sprinters, differing in handle shape and color, and differeing measuring, gradiated tip, they are all longer than "necessary" and -as far as I've been able to tell - built like a flexible cable/spring with a pointy, tip. The tips either have 6 hash marks or two.
I am wondering if they sold you the wrong dipstick.
For reference, about 45 1/2 inches from the top of the dipstick tube to the bottom of the pan (not a precise measurement). I've marked mine (pictured) with black tape after I dropped the pan, poked the tool down through the dipstick tube until the measuring portion protuded to the depth of the bottom of the pan.
-Jon