Mi ĉiam havas la kvitancon en mia sako, se la vendejo demandas pri ĝi poste.

Because kvitancon is the direct object of havas.

• Mi havas la kvitancon = I have the receipt
• The thing being had is the direct object, so it takes the accusative ending -n

The basic form is kvitanco. In this sentence it becomes kvitancon.

La means the. It shows that the speaker means a specific receipt, not just any receipt.

Here, the idea is probably the receipt from that purchase, so Esperanto uses la just as English does in the receipt.

In Esperanto, possessive words like mia, via, lia, ŝia, and so on usually replace la.

So:

• mia sako = my bag
• not normally la mia sako

The possessive already makes the noun definite enough.

Because this sentence describes location, not motion.

• en mia sako = in my bag (it is located there)
• en mian sakon = into my bag (movement into it)

Here the receipt is already in the bag, so en mia sako is correct.

Because pri is a preposition, and after a preposition you normally use the basic form, not the accusative -n.

So:

• pri ĝi = about it
• not pri ĝin

The -n ending is mainly for direct objects, but ĝi here is part of the prepositional phrase pri ĝi.

Demandi pri means to ask about something.

So:

• La vendejo demandas pri ĝi = The store asks about it

This is a very common Esperanto pattern:

• demandi pri la prezo = ask about the price
• demandi pri la problemo = ask about the problem

Because ĝi is the pronoun for a thing or non-person.

The noun kvitanco is an object, not a person, so ĝi is the natural pronoun:

• kvitanco → ĝi

So pri ĝi means about it, where it refers back to the receipt.

Because se means if, while kiam means when.

• se la vendejo demandas = if the store asks
• kiam la vendejo demandas = when the store asks

Using se shows that this is a possibility or condition, not something the speaker is treating as certain.

Demandas is present tense, but Esperanto often uses the present tense for general, repeated, or habitual situations.

This sentence has that kind of meaning:

• I always keep the receipt in my bag, if the store asks about it later

So demandas works well for a general rule or habit.

If you said demandos, that would sound more like a specific future case:

• ...se la vendejo demandos pri ĝi poste = ...if the store will ask about it later

Both can make sense, but demandas fits a general statement nicely.

Because ĉiam is an adverb, and putting it before the verb is a very natural position in Esperanto.

• Mi ĉiam havas... = I always have...

Esperanto word order is fairly flexible, so Mi havas ĉiam... may be understood, but Mi ĉiam havas... is the most natural choice here.

Poste means later or afterward.

In this sentence it modifies the idea of demandas:

• se la vendejo demandas pri ĝi poste = if the store asks about it later

Putting poste at the end is very natural. Esperanto often puts time adverbs in a flexible position, but the end of the clause is common and clear.

Yes. This is normal shorthand.

La vendejo demandas really means something like:

• the store staff ask
• the store asks
• the shop contacts me and asks

English does this too: The bank called me, The school wants a form, and so on. Esperanto allows the same kind of expression.

Yes. Esperanto word order is fairly flexible because endings show grammatical roles.

For example, this would also work:

• Se la vendejo poste demandas pri ĝi, mi ĉiam havas la kvitancon en mia sako.

That version puts the if clause first. The meaning stays basically the same.

The original order is still perfectly natural:

• Mi ĉiam havas la kvitancon en mia sako, se la vendejo demandas pri ĝi poste.