Customization of Checkout Process

How do I make the default option:

I will use my existing domain and update my nameservers rather than Transfer your domain from another registrar?

Having a lot of trouble figuring this one out...

I would assume the easiest way would be to alter their order in the template - which order form template are you using?

I'm currently using the default template - I tried to switch the order, but that did not work...

The location of the template is: /home/<REDACTED>/domains/whmcs/templates/orderforms/modern/configureproductdomain.tpl

I'm currently using the default template - I tried to switch the order, but that did not work...

are you sure?? I just tried it and it worked fine..

in configureproductdomain.tpl, there are four {if} blocks which control the order in which they're displayed (lines 25-65)...

{if $registerdomainenabled}
...
{/if}

{if $transferdomainenabled}
...
{/if}

{if $owndomainenabled}
...
{/if}

{if $subdomains}
...
{/if}

all you should need to do is move the blocks of {if} code into the specific order you want to remove - so in your case of using the client's domain first, you would change the order as...

{if $owndomainenabled}
...
{/if}

{if $registerdomainenabled}
...
{/if}

{if $transferdomainenabled}
...
{/if}

{if $subdomains}
...
{/if}

if you do that, then you should end up with the screen below.

http://prntscr.com/2x77dx

possibly you may need to empty your templates_c folder and/or browser cache to see the change, but it should work.