invoices and daily receipts

No, an invoice is indeed automatically added to your income (and profit).

Do not double count it, so do not enter it as both a sales invoice and a daily receipt.

You should read the article about daily receipts, then you will understand what it is :) Daily receipts are also turnover (income) but in a other way than a regular sales invoice.

And if you work with someone/a company that pays you by invoice, how do you enter that?

If you still use a paper 'receipts' book, can you just enter the monthly total in the online section?

But with a company... Then it's not allowed anyway, daily receipts are only for private individuals.

I think so, for that paper book. Ultimately, you've kept track of it good in the paper book and then just stopped the turnover in Dexxter. So I think it's okay in that case.