'Warning: Cannot modify header information .....' Error ?

I am trying to set up the first part of an Application process where a payment is required so I am using Ecart, incidentally I will be requesting help via a support ticket once I have taken the project as far as I can.

( Ray might recall helping me back in May for a membership application using Ecart for 'lease-assn.org' which was similar to this project.)

I have so far:-
Created an Ecart object.
Added the 'Add to Cart' behaviour to the submit button of my Application form.
I have created a Display Cart page with a 'Checkout Button'
And last of all I have run the 'Checkout Wizard' to create the pages.

On testing I am submitting the application then clicking the 'Checkout button' and it directs to the checkout page with an error as follows: -

Warning: Cannot modify header information - headers already sent by (output started at E:\Domains\r\rtmf.org.uk-20218885\user\htdocs\checkout_f.php:138) in E:\Domains\r\rtmf.org.uk-20218885\user\htdocs\confirm_f.php on line 116.

I am pretty sure there aren't any white spaces between tags so what else can cause this error?

Subsequently I thought also because I do not require shipping it should skip this page As this is what happens in my previous project.

Ive attached the files I thought might be relevant.

Thanks in advance.