I say very clearly in my listings that postage discounts are only given on items paid for on a single invoice and that if they are paid separately then they will be sent separately.

I never send a reminder as such as ebay send one ofter 48 hours anyway. If they haven't paid by the end of day 3 I send the invoice with a message asking them for details of their payment as I have been unable to trace it, a kick up the posterior in polite lingo lol.

If still nothing after the end of day 4 I open the dispute.