If there is no Erase button (Which I'm assuming you would of seen) then the only way is to wait for it to 'Expire' or write more posts on the board until it goes off.
You can't
You press the letter and somewhere it will say 'toss' and then it will say 'toss' again and then you have to click on that and it will disappear PS you can only toss letters when you are outside!!!
When you talk to the animal that gave you the letter, he/she will be mad at you.
you can't
It is impossible to send a letter to any of the shops/buildings in the town and/or city in Animal Crossing City Folk. Letters can only be sent to other town folk.
no you cant 9 (sadley)
yes
Go to Tom Nook's Store, they come in a pack of 4.
you get a letter from mom. Inside of it, there are pink carnations
No, unless your actual mother has a character in the game.
You cant because she is not in your town.
No sorry I'm afraid you can't. But what you can do is maybe let Coco come to your place or you visit her and maybe you can act as if its a date there or you could go for a walk with coco.