Learn Hardcore Dutch: De koerier levert de boeken meestal soepel en op tijd. - The courier usually delivers the books smoothly and on time.

In Dutch, de is the definite article for most singular common-gender nouns and all plurals. Here it appears before koerier (singular, common gender) and before boeken (plural).

de koerier = the courier
de boeken = the books

What does leveren mean, and how does it behave with objects?

Leveren means to deliver. It’s a transitive verb that can take one or two objects:

With a single direct object:
De koerier levert de boeken. (The courier delivers the books.)
With direct + indirect object:
Hij levert de klant de boeken. (He delivers the books to the customer.)
In our sentence we only have the direct object de boeken.

What does meestal mean, and where can it be placed in a Dutch sentence?

Meestal means usually or most of the time. Dutch is a V2 (verb-second) language, so the finite verb must be in the second slot. You can put meestal either:
1) Directly after the subject (verb still second):
De koerier levert meestal de boeken…
2) In first position (verb moves to second):
Meestal levert de koerier de boeken…
3) After the object, for emphasis on manner/time:
De koerier levert de boeken meestal…
All are grammatically correct; choice depends on what you want to emphasize.

Why is soepel used without any ending, even though it looks like an adjective?

When a Dutch adjective is used adverbially (to modify a verb), you keep its base form. Here soepel (normally smooth or flexible as an adjective) functions as an adverb meaning smoothly. If you used it before a noun, you would add –e:

Adverb: De koerier levert soepel. (The courier delivers smoothly.)
Adjective: een soepele levering (a smooth delivery)

What does op tijd mean, and why not in tijd?

Op tijd is the fixed expression for on time or punctually. You can’t say in tijd for this meaning. If you wanted in time (as in in time to catch the train), you’d still use (net) op tijd or another structure like tijdig.

Why are the adverbs soepel (manner) and op tijd (time) in that order?

Dutch generally orders adverbials as follows: manner → place → time. Here soepel (manner: how?) comes before op tijd (time: when?). Switching them (op tijd soepel) sounds unnatural.

What’s the difference between meestal and vaak?

Meestal = usually, most of the time (implies a regular pattern)
Vaak = often, frequently (focuses on frequency)
You could say De koerier levert vaak… to stress that deliveries happen often, whereas meestal suggests they happen that way in the majority of cases.