Whenever you see an error message about the transaction not going through it is probably because of an error in the transaction that is not being shown on the screen.
You can add response bindings to your failure page to show the error from the gateway, this will help to determine what the error is and what is causing it.
We have a PDF document that details how to add these responses to your pages:
This can be found in the support page for eCart under How To's:
There is no option in eCart to set the max limit of the transaction.