even after I sent many invoices
In future, send one invoice, or even let eBay send it for you.
On Day Three, send a polite reminder.
On Day Four open the Unpaid Item Dispute.
Less communication is better.
The ones who don't pay at all are deadbeats and winning a case against them means, among other things, that the deadbeat cannot leave feedback.
The ones who need a Dispute to kickstart payment are just slow payers.
Unless you have had acrimonious correspondence with them, there is no reason to think they are anything but a bit vague about paying bills.
Ship.
If you did get grumpy with the buyer, you might want to use tracking to stop some unwanted new disputes.