Learn Arabic (Modern Standard): عندي ستون دولارا وثمانون طابعا.

In Arabic, numbers from 11 to 99 are always followed by a singular noun. Even though we say sixty dollars in English, Arabic grammar requires you to literally say sixty dollar.

Why do دولارا and طابعا end with an extra alif?

The extra alif carries the -an ending (tanwiin fatha), which marks the accusative case. Any noun counted by numbers from 11 to 99 must be both singular and in the accusative case.

Why do the numbers ستون (sixty) and ثمانون (eighty) end in -ون (-uun) instead of -ين (-iin)?

The -ون ending marks the nominative case (used for the subject of a sentence). When you use عندي (at me) to say you have something, the things you own are actually the grammatical subjects of the sentence. Since they are the subjects, they get the -ون ending.

Is there a reason we use عندي here instead of another word for to have?

Arabic doesn't have a direct verb for to have. Instead, it uses prepositions like عند (at/with) with attached pronouns. عندي (at me) is the most common way to talk about owning physical items like dollars and stamps. It never changes based on what you own!