Dealing with larger 'partners'
"The larger the 'partner', the less flexible and more unreasonable they will be."
Isn't that just so true. At my last job we were a relatively small manufacturer and wholesaler - really only "back of the sofa" cash to the larger retail chains. "discussion" regarding EDI went along the lines of "you will be doing X ... or we won't be dealing with you<period>"
I could see the benefits, but out ERP system didn't support them very well unless we spent a fairly large wedge of cash with the vendor on additional modules - and even then we would not be able to meet some requirements. Quite common was for a customer to require specific that effectively made our "common" EDI setup useless - at one point I think we ran no less that four "off the shelf but customised" installations of the same EDI package in order to trade with four different customers who all wanted something different. The end result was that the customers got all the benefits, not by any overall reduction in work, but by making us do their back office work for them.
On this last point, in all these cases, our staff had to effectively process everything twice - once on our systems to control our stock and finances, and once on the customers EDI systems to enter their backend data. A small number would send us orders, and accept invoices, in standard formats we could deal with - for them we could save effort overall, but others just insisted on customer specific extras that meant we couldn't use any existing system.