Should I give an opportunity to give in every letter?

No. Though some ministries stick a courtesy pledge card in every newsletter it might be counter-productive.  The donor already receives an official receipt (I hope!) and that receipt package has a turnaround form for future giving (I hope!).  If your organization does not provide that service then you will need to insert a receipt and turnaround document for the next gift yourself.

For most cultures I recommend you only make an appeal for your general budget once a year and once more for a special project (if you have one) such as a trip to another country or hiring a new staff, etc.  In your regular quarterly or bi-monthly newsletters say thanks only and nothing else about funding.