The first thing I would be doing is to ring the bank this morning and confirm if the cheque has cleared or not. If you paid Weds then it may have been put in on Thursday or even Friday. It is unlikely to have cleared over the weekend and you may be able to stop it. If they don't get paid then they will be more likely to listen to any concerns you have. Andy's advice is sound in use your credit card for payment if you get the opportunity. I have fitted large french doors with side panels. Yes they can flex due to there given size but usually overcome with fixings through the head of the frame. If they have had to pack it with frame extensions then the fixing is probably flexing as well assuming that they could be bothered to secure the head. As TNP said expanding foam is the answer and will remove ad flex. So in short, check on payment and then either get a new snug fitting frame or compo if you can live with the one you have got and get it foamed to remove the flex. I assume that there are no low fascia's on the outside and that the frame has not been deliberately made smaller so that the doors clear any possible obstructions?
Regards Legin