Conditionals on Invoice e-mail

Hello,

I would like to create a conditional on invoice template like this,

{if $invoice_total bigger than "1000"}
Please pay using this payment method
{else}
Please pay using paypal
{/if}

In other words, if the invoice total is bigger than 1000 I would like to display one payment method, and if not, I will display paypal link. But it seems that WHMCS is not accepting "bigger than". What can I use instead ?

Thank you.