Customer can't pay with Amex

I'm really sorry but I'm posting this issue again as it still persists. 

A customer of mine from a company based in the US can't pay me with her Amex. I'm not sure if there's supposed to be but there is no option on the payment page to choose Amex or Visa. 

Last week someone from MYOB was assisting me but they've passed it to someone else and I haven't heard anything in around a week with no way of getting in touch. 

If anyone has heard of or is having the same issue please let me know. 

Alternatively if there is a number I can call I would appreciate the help as I can't seem to find any numbers listed

Graeme