If MYOB is having problems with the emailed invoices, perhaps you could print to a PDF (using something like
PDFCreator) and then manually email the customer the file . That could certainly be done while you are out without the need to return to your home office. I could see you sending the invoice while at the customer site, and asking them if they got it before you leave the site and without having to go back to your car for the printout.
It would manually achieve the same thing as the MYOB facility - and lave you carrying on less thing as you visit your customers...