There are two different し here. The first is the particle し, which can be used to list verbs/sentences in a way that is analogous to the way と
is used when listing nouns.
雨がふっているし、車がこしょうしているし、あかちゃんがないているし. . .
"It's raining, the car's broken down, the baby's crying . . ." There's no theoretical limit to the number of times you can use this particle in a list of sentences, but I'd say it usually comes in ones, twos or threes.
The second し, following 予約も, is simply the first syllable of して, the て form of する, in the pattern してある, "has been done".
ホテルの予約がしてある would mean "the hotel reservation has been made". Replace が with も and you have "the hotel reservation has been made, too", "the hotel reservation has been made as well".
So the whole means "Mm, I've bought the tickets, the hotel reservation has been made . . . [and so on]". I'd say the use of し suggests that this is the first in a list of things, so there are other items that the speaker might have mentioned - "I've bought a new suitcase, I've cancelled the mail. . . "