Learn Hardcore Turkish: Biletleri de ben alıverdim; artık sadece gitmek kaldı. - I went ahead and bought the tickets too; now all that’s left is to go.

gitmek kaldı: generic/impersonal “(only) going remains,” with the participants understood from context.
gitmemiz kaldı: “our going remains,” explicitly tying the action to “us.” Very natural if you want to be clear: Artık sadece gitmemiz kaldı.

Breakdown of Biletleri de ben alıverdim; artık sadece gitmek kaldı.

What does the clitic de do here, and why is it written separately?

de means also/too/as well and links the clause to something previously mentioned (e.g., “I also took care of the tickets”).
It attaches to the phrase it follows, so Biletleri de means “the tickets, too.”
It’s written as a separate word (unlike the locative suffix -de/-da as in evde).
It follows vowel harmony as de/da, but it never becomes te/ta (that only happens with the locative suffix).
Here, biletleri ends in a front vowel, so we use de: biletleri de.

Why is the pronoun ben present? Isn’t the subject usually dropped in Turkish?

Yes, subjects are often omitted because the verb ending shows person.
Using ben here adds emphasis: I (as opposed to someone else) did it.
The placement right before the verb (… ben alıverdim) puts ben in the focus position, highlighting that the speaker personally took care of it.

What’s the nuance difference among these orders?