It sounds like now is the time to learn to be a good boss. They know you're crap and they are taking the piss, probably convincing themselves that because you don't give them work, it's ok to lounge around.
It's really not that difficult. For a start, Skype meeting at 9am assuming that's when they would otherwise need to be in the office.
Discuss what has been done the day before and what hasn't. Say it as it is, that you know some of them (don't give names) were not logged in after 12pm, and you want them to email you to let you know why that was so you can address the problem. Remind them that it is the expectation that they work the hours they are paid to work, and if they run out of work, they should be contacting you to let you know. Only you can authorise them to log off.
Your point about scanning taking as long doesn't make sense if you have half of the staff doing nothing and half overworked. Get the staff in the office to prioritise scanning and sending, and those at home to deal with the invoice.
Ask all your staff to complete a handover at the end of the day with which invoices they've dealt, which are outstanding and what is left to do.
You can't be a manager and not take control of your team. Many people will self-motivate themselves to work and find things to do, but many will use the excuse of being at home and not 'watched' to consider their work is to just do what comes their way and not look proactively at what else they could do. Take into account the mental impact of the lockdown and people will feel even less mentally motivated. It's your job to motivate them and remind them what you expect of them.