Iād like to elaborate a little bit. Sandals and flip-flops really mean the same thing in some contexts. Flip-flops is sort of slang for sandals. Flip-flops are basically named after the sound they make. āFlip-Flopā with every step.
In summer refers to the season in general. In the summermay also do so, but it can also refer to a particular summer. So, the sentence is perfectly correct as @Alan wrote it.
Exactly as pointed out by Alan. Reread that screenshot message. I donāt think it necessary for me to explain it to you again. Alanās explanation is very good.