Could not finish your order

Hello

How can I find out why my test orders are failing on scFinishOrder with SimpleCart v2.5.2

I have the following on the step before:

<input type="hidden" name="doOrder" value="true" />
<input type="hidden" name="deliveryMethod" value="1" />

because the orders are free and delivered electronically so ideally I want to skip through delivery options and payment.

I have looked in the error.log and can see:

[2018-10-15 22:00:47] (ERROR @ /core/components/simplecart/controllers/web/Cart.class.php : 1605) PHP warning: Division by zero
[2018-10-15 22:00:47] (ERROR @ /core/components/simplecart/controllers/web/Cart.class.php : 1605) PHP warning: Division by zero
[2018-10-15 22:01:18] (ERROR @ /core/components/simplecart/controllers/web/Cart.class.php : 1605) PHP warning: Division by zero
[2018-10-15 22:01:18] (ERROR @ core/components/simplecart/model/simplecart/simplecart.class.php : 267) PHP warning: number_format() expects parameter 1 to be float, string given
[2018-10-15 22:01:18] (ERROR @ /core/components/simplecart/model/simplecart/simplecart.class.php : 267) PHP warning: number_format() expects parameter 1 to be float, string given

(paths have been shortened in the log file by me before pasting)

Any tips on how to debug gratefully recieved.

James