I mean how is it different to ordering anything else and doing a measure yourself? Like I said, it's a inside window frame fit, it's not really that hard to get the measurements wrong. This is a DIY forum after all - have you never provided measurements for anything yourself without having someone come and do their own measure?
I'm just saying have you got in contact with any companies you looked at and asked what they will do? I'm sure they're more than willing if you simply asked and you could negotiate a discount if you think their price includes them coming out.
Of course on their websites they need to prepare for people who can't DIY at all.