Thanks a lot Gearhead!
I've been doing a lot of experiments with "Invoices" template emails, but I'm not getting the expected results. Even with a very simple conditional, like {if $invoice_payment_method eq "Paypal"}TEXT{/if} or {if $invoice_payment_method eq "Paypal"}TEXT{else}NOTEXT{/if}), I'm getting bad results (TEXT is showing when it shouldn't, when the payment method for the invoice/customer I'm using for the tests is not Paypal).
I'm not sure if I'm doing something wrong with the conditionals code (I'm putting it in the source code and it seems OK, but who knows...), with the customer account I'm using for the tests (it's not a real account, it's services are cancelled actually) or something else.
I'm using {debug} and when I change the payment method in the customer account it reflects OK in the variable (I see the change in the debug popup), but in the email itself I don't see what I expect.
Any clue of what could be the problem? Should I send an email to WHMCS support for this?