Buying a bike with Shimano Ultegra Electronic shifting.

[MarieV]

I've had Di2 on one of my bikes. You use the same Ultegra 165 mm crank for Di2 and mechanical. The electronic part only refers to the shifters and front and rear derailleurs. The crank, cassette, and chain are exactly the same as the Ultegra mechanical.

It's totally worth it to change the crank to the 165 mm crank since you're shorter. However, 165 mm cranks are usually more expensive since Shimano makes so few of them, which may be why Trek and other bike manufacturers put the more common and cheaper 170 mm cranks on their smallest bikes and probably why your LBS isn't willing to swap the 170 cranks to 165 cranks at no charge. I'd ask your LBS if they're willing to swap the crank out for the difference in cost between the cranks.

[Crankin]

This is a crock. My LBS said there was a back order/delay on getting the 165 cranks, but they never discouraged me. In the end, I got the bike about 6 weeks earlier than they had thought! It is always worth it to pay for this upgrade. On my trip to Provence, I was riding a very nice Madone from 2014. It felt almost exactly like my Silque, except for the 170 cranks. My choice was to ride this bike, or a 44cm, with the 165 cranks. The 44 cm would be too small for me, so I sucked it up. My knees have not been right since the trip, and I don't have knee problems.
This sounds like when I bought my Kuota in 2006. They built it up because the shop owner (different LBS) said the 170 cranks would "turn me into a climbing animal." This is just old fashioned male racer BS. I never stand to climb and with my 165 cranks, I do just fine for a 61 year old.

[Tokie]

Thanks everyone.I haven't gathered my energy to negotiate with the local dealer.And if he isn't willing,then there are bigger Trek dealers in nearby towns. I used to have 170's- yes you get more leverage,but it isn't right for me either Crankin. thanks, Tokie

[OakLeaf]

Yeah, I'd look at it more like a bigger dealer is more likely to be able to sell the takeoff AND more likely to be able to absorb the loss if they can't sell it. Unlike some other components that I'd expect a shop of any size to swap out at no charge (or the difference in cost if there is one), cranks don't get replaced very often. Bike shops usually operate on a very narrow margin. Besides looking for a bigger shop that moves more volume, I'd look for a shop that does more custom builds than complete bikes, which a shop like that would also be more likely to do that for you even if it's a smaller shop.

I can't ride anything longer than 165s without hurting my knees either.