Learn Farsi: یا زیپ را انتخاب کنید یا دکمه را.

It translates to 'either... or...'. In Farsi, you place yâ right before each of the options you are choosing between. Here, the options are zip (zipper) and dokme (button), so yâ goes directly in front of each.

Why is the object marker râ used twice?

râ marks a specific direct object. Since you are choosing between the zipper and the button, both are specific objects. When using a paired conjunction like yâ ... yâ ... in Farsi, it is standard to repeat markers like râ (or prepositions like az and be) after each option to keep the sentence balanced.

Why is the verb entexâb konid in the middle of the sentence instead of at the very end?

While Farsi verbs normally go at the end of the sentence, using correlative conjunctions like yâ ... yâ ... gives you some flexibility. It is very common and natural to place the second option after the verb so the list of objects doesn't get too clunky. You could also put the verb at the very end (yâ zip râ yâ dokme râ entexâb konid), but this split word order is widely used.

This is a command to 'choose', so why doesn't the verb have the imperative prefix be-?

entexâb kardan (to choose) is a compound verb formed with kardan (to do). When making commands with kardan verbs, you generally do not add the prefix. You simply take the present stem and attach the personal ending, giving us for the formal or plural 'you'.