I am a Small business owner, actively looking for an online invoice system. I need to invoice my clients every week approximately 10 invoices a week. Can anyone suggest me a good online invoice system which is not that costly or free  

I am not affiliated with SquareUp or Square as they are called but as a small business owner I have been impressed with what they have done in the last 5 years. Try this link to learn about invoicing with them, https://squareup.com/invoices .

In today's world, you really do NOT want to rely solely on someone sending you a check in the mail, if you can help it... for your sake and for theirs.

I didn't mention Quickbooks online because if you are asking this question, it is assumed you already know about them and decided you wanted something else. Maybe Quickbooks wants to think that it "defines" bookkeeping, and perhaps it does, but there are other services that have made inroads and at a cheaper price. Square is one of them, maybe that is the reason that SO many small retail stores use their service and point of sale stuff. You should check them out. Good luck

I will say Quick Books,
It is super easy, and I have used it for over 12 years

Another option you might consider is Freshbooks ( https://www.freshbooks.com/ ). I've not used it myself--and I have no financial interest in it--but I've heard some good things about them.
Good luck,
WallyH