The first one is more polite. But they both sound good and make sense. Hope this helps!

Hi Ali! Both are polite and natural. Exactly as Anne said, "could you please" is a little more polite than "can you". However using "please" at a gas station is already very polite so it's not necessary. Personally I would not say "$20 worth". It's a very small thing and it makes sense! However I would say "Can I have $20 of gas, please?"/"Can you give me $20 of gas, please?" Mich https://www.italki.com/michelle

Please, can you give me 20$ worth of gas? The sentence above is polite and simple Ali. I hope that answers your question. If you'd like to practice speaking, I'd be happy to help you develop a study plan. Feel free to look at my teaching profile and class offerings and send me a message if you think I can help you.

Thank you

Hi Ali, I would suggest saying "Could you give me $20 worth of gas, please?". "Could" in this context is the polite form of "can". https://dictionary.cambridge.org/us/grammar/british-grammar/can-could-or-may