Hello @userpaintpotsofdevon,
There are times that your browser is full of frequently-accessed page resources, thus causing websites, like QuickBooks Online (QBO) to act weirdly. This can be the reason why you're unable to send your customer's invoice successfully.
Let's consider accessing the website and logging into your account using a private browser so we can isolate this issue. To do so:
- Google Chrome: Ctrl + Shift + N
- Mozilla Firefox: Ctrl + Shift + P
- Safari: Command + Option + P
If this works, you can clear your regular browser's cache so the system can start fresh. But if you get the same results while using a private browser, use a different browser app instead.
Additionally, I've also included this reference for a compilation of articles you can use while working with us: Sales and customers for QuickBooks Online.
If there's anything else that I can help you with, please let me know in the comments sections below. I'll be here to lend a hand.
