minimum
Iota
A line where one place or thing starts And another ends. The line between states
Iota.
Headache starts and ends with the letters he. Another word is heartache.
donk
Tremble
Idle
alter
minute, little, petite
The answer to the riddle is "Mailbox." It starts with the letter 'M,' ends with 'X,' and can contain an endless number of letters (as in mail) inside it.
Autumn. You lied about the b.
"TOURIST"