I was looking for London to Buenos Aires business class return through Avios with an Amex 2-1 voucher (half price rather than 2-for-1). I couldn't find seats LHR-EZE but found some LHR-MVD which was close enough. I don't have the exact figures to hand but it was around 75,000 Avios and £550 which I was happy with.

I then discovered that open jaw tickets are available if you ring and this worked for me LHR-MVD then EZE-LHR (Iberia via MAD). When it came to payment I was quoted 75,750 Avios and £1,300+. I thought that this might be because of the open-jaw flights but asked him to check. After 10 or so minutes on hold he quoted the same Avios and £478 fees. I was working and had spent long enough on this so regretfully I didn't query why the difference.

I'm simply posting as a warning to anyone else that might be calling to book tickets in this way. In my rush to get the call over with I was tempted to just pay it but I thought that I'd prefer to go in and out of Montevideo at the expense of a few hours travelling but for a significant saving.