It sounds like your buyer is probably using a mobile app, which doesn't facilitate multiple items from being combined on a single payment, even after an invoice has been issued.

There's a couple of workarounds, depending on what's available to the buyer - I believe (though haven't tried it myself) that if they go to their email and click 'Pay Now' on the emailed invoice, it will bypass the app checkout and keep all items combined (they can also just switch to a browser instead of the app, and click to the classic site version of eBay, in order to check out - jumping on a PC will also work).

Failing any of those options, you can send an invoice directly through PayPal for the combined amount.

If you have set up a PayPal Me profile, you can just send them your URL with the amount after a forward slash, eg if your PPM user name is mitchoc59 and the total due is $25, then the URL would be paypal.me/mitchoc59/25 (it's a lot easier to send the URL via eBay messages than it is to send invoices / exchange email addresses, though you would already have the buyer's PP address if they've submitted a payment). I always advise the buyer that they may need to select the payment is "for goods" and that I'll update the payment status on eBay as soon as it comes through.