Hello there @savvasst and thanks for joining in on this thread to share your thoughts regarding the default numbering system used in QuickBooks Online for sales transactions.
As @LauraAB explained in her reply above, both Invoices and Sales receipts follow the same numbering system in QuickBooks Online. If you create an Invoice that is numbered 100, followed by a Sales Receipt, the Sales Receipt will be allocated number 101. If you'd like to use a different numbering system, you can manually override the number in the field. Ensure that you're keeping track of your numbering system elsewhere so that you know the next one that should appear in the sequence for each transaction type.
I'd also like to encourage you to submit feedback on this in-product. When you let us know how we can improve the program, we take those suggestions into consideration when implementing new features.
Take care and have a great weekend!
