"We will have sent the letters."

Translation: Vi kommer att ha skickat breven.

[JeanLibera1]

Is this possible: Vi ska ha skickat breven.

[frrro]

No, it would mean "should have" (I'm native)

[Onoszko]

Att utelämna "att" är numera helt okej.

[Zmrzlina]

Yes, for most cases where it is optional, it is accepted with or without the att.

[ThomasGard82099]

My Swedish girlfriend is a muppet