Inserting Online Payment Link into body of invoice

Re Invoices - is there any way i can insert a link to online payment on the invoice itself? It's included in the covering email which is great - but many of my clients (large organisations) file the invoices away after they've received them and when the invoice is eventually due to be paid, it is no longer attached to the covering email - and then there's no way they can pay online.

So just wondered if i could possible insert the online payment link into the invoice itself? Have tried all evening to do this but I can't get it to work.

Many thanks

Trish (TSA)