Learn German: Wir bezahlen insgesamt sechshundert Euro. - We are paying six hundred euros in total.

In German, numbers up to one million are always written as one continuous word. You just attach hundert (hundred) directly to the single digit sechs (six) to make sechshundert.

Why isn't the word Euro pluralized to "Euros" since the amount is more than one?

Masculine and neuter nouns that express a unit of measurement or currency do not take a plural ending when they follow a number. Therefore, the amount stays sechshundert Euro rather than "Euros". You might only hear Euros if someone is talking about the physical coins themselves, not the monetary amount.

Could I use zahlen instead of bezahlen in this sentence?

Yes! Both zahlen and bezahlen mean "to pay" and are often interchangeable. Bezahlen sounds slightly more like you are settling a specific bill, while zahlen is more general, but both work perfectly here.

Why is insgesamt placed exactly where it is in this sentence?

The adverb insgesamt (in total) typically comes right after the conjugated verb (bezahlen) and directly before the specific information it describes, which in this case is the total amount (sechshundert Euro).