It can depend on the reason sometimes.

I would certainly issue a letter and have a verbal discussion. I always make sure I have a cheque in hand before I terminate. So, if they pay at pick up I would take the money then have the conversation, give the letter and let them know if they have any questions you are happy to discuss over phone.

Do you policies cover a termination period? ARe you obligated to carry on for 2 weeks?

If you are discussing issues or progress daily then it shouldn't be out of the blue for them.

Can you elaborate on the issues your having? And what brought you to this desicion?