"Please keep in mind that this code can only be used once within the next 60 days, and any remaining balance is non-refundable" - Is this sentence proper? Can it be argued that "once" can be used as a conjunction here?

I know nothing about grammar; however, below is my argument from the research I've done. I believe this company was trying to fool me with the way they worded the sentence.

The main issue here is using the word "once" followed by "within" and "next". "once within". o"Once", as a conjunction, means "as soon as", "within" signifies "inside or not further than an area or period of time" - cambridge.com so "once within" would mean as soon as the code is inside that period of time I'd be able to use the code. That would mean I could use the code the day after it was given to me (next) and it would be valid for 60 days... So I'd be able to use the market credit code balance as many times as I wanted during that 60 day period until it ran out. If the market credit is not used up in that time period the remaining balance would be