Umbúðirnar eru ekki til staðar, því miður.

-nar is the attached definite article for feminine plural nominative nouns.
So umbúðir = packaging/wrappings (indefinite), and umbúðirnar = the packaging / the wrappings (referring to specific ones).

The lemma is usually given as umbúð (singular, feminine).
Common forms:

• Singular: umbúð (indef.), umbúðin (def.)
• Plural: umbúðir (indef.), umbúðirnar (def.)

Icelandic often uses the plural where English might use a mass noun like packaging. umbúðir commonly refers to packaging as a set (box + wrapping, etc.).
You can use singular umbúð if you mean one specific wrapper/package, but the plural is very natural for “the packaging” in general.

Because the subject Umbúðirnar is plural.

• er = is (3rd person singular)
• eru = are (3rd person plural)

Yes, til staðar is a set phrase meaning present / on site / available (in a place). It’s extremely common with vera:

• vera til staðar = to be present
• ekki vera til staðar = to not be present

Literally, til often means to/toward, and staðar is a form of staður (place). But you don’t normally translate it word-for-word; treat til staðar as an idiom meaning present. It’s closer to English on hand than to to a place.

staðar is genitive singular of staður (place). The phrase is fossilized/idiomatic, so it’s best learned as a chunk: til staðar.

In neutral main-clause word order, Icelandic typically places the negation ekki after the finite verb:

• Þeir eru ekki hér. = They aren’t here.
  You can move ekki for emphasis in some structures, but eru ekki is the default pattern.

því miður is an idiom meaning unfortunately. Literally it’s something like for that, sadly. It’s used as a sentence adverb/comment, similar to English unfortunately / I’m afraid.

Not strictly, but it’s common. The comma signals that því miður is a parenthetical comment. You’ll also see it without a comma in more flowing text, but the comma is very normal in careful writing.

Yes. Common placements include:

• Því miður eru umbúðirnar ekki til staðar. (more fronted/emphatic)
• Umbúðirnar eru ekki til staðar því miður. (more casual)
  The meaning stays basically the same; placement affects emphasis and rhythm.

You can, but it changes the feel:

• Umbúðirnar... = the (specific) packaging (both speaker and listener know what packaging)
• Umbúðir... = more general/indefinite (packaging isn’t present / there is no packaging available), and it can sound less specific.

• þ is like English th in thing (voiceless). So því starts with that sound.
• ð is like English th in this (voiced) in many environments, though it can be softer or even disappear depending on position/speed. In umbúðir, the ð is typically a voiced th-like sound.

If you want, tell me your accent (US/UK/etc.) and I can give a closer pronunciation guide for the whole sentence.