Adding variable in custom field


I have a custom field on my invoices, named “Remarks”.
In this custom field, I need to get the total amount of the invoice.
Why? Because I want to give my clients the ability to pay also via Paypal.
With the short link[my-paypal-account]/[total amount of the invoice], the client will be referred to my paypal account where the total amount to pay is already filled out. Then he just has to click next.
But I can’t find how to get the total amount as a variable in my custom field “remarks”.
{{ table.totals[“Total”] }} does not give me the amount but the text " {{ table.totals[“Total”] ".
And is it also possible to get the same variable in my e-mail template?
Thanks for your answer!

I’m not sure if it’s possible to get that information into a custom field - you may be going about this the wrong way.

Instead, take a look at creating custom themes for your invoices: Change the look of forms with themes | Manager (scroll down to “Adding custom themes” sub-heading). Support for custom themes is generally beyond the scope of this forum, however, so if you have any detailed questions about custom theme syntax (HTML / CSS / Liquid) you may need to hire someone to help you.