Learn French: J'ai acheté la crème solaire et la batterie externe que tu as demandées. - I bought the sunscreen and the external battery that you asked for.

This is a classic tricky rule in French! Usually, past tense verbs using avoir don't change their endings. However, if the direct object comes before the verb, the past participle must agree with it. The direct object here is la crème solaire et la batterie externe (two feminine nouns, making it feminine plural). Because the word que refers back to them and is placed before the verb, we add -es to demandé.

Why is there no French word for "for" at the end of the sentence?

In English, we say "to ask for something," but the French verb demander inherently means "to ask for" when referring to an object. Because the preposition is built into the verb's meaning, you don't need to add a word like pour.

Could I just say la crème solaire et batterie externe without repeating la?

No, in French you generally must repeat the article (like le, la, or les) before every single noun in a list. Even though both items are feminine, you need la before each one.

Does crème solaire literally mean "solar cream"?

Yes! Solaire is an adjective meaning "solar" or "relating to the sun." Like most French adjectives, it is placed after the noun, giving us "solar cream" for sunscreen.