I want to buy the m2 air from Singapore. Do I have to book an appointment before purchasing from the store? And as an international student outside of Singapore, can I get the education discount? Most importantly, what are the important documentation I have to bring with me to avail the discount?
In the US, no appointment is needed to make a purchase at the Apple Store.
Regarding the proof students need to provide, a photo ID from the school is probably the best proof of enrollment. A course enrollment form and/or valid .edu email address will often work as well.
As far as not being local to Singapore, yet wanting to purchase there, you should probably contact Apple directly and find out from them. Contact Apple for support and service - Apple Support
in-store purchase and education discount
