Using the present tense for future events is normal usage in English, when the event is certain. As you tell your interlocutor to "make sure", using the present reinforce the certainty of that happening, there's no "maybe".
See the article on the British Council
> When we know about the future we normally use the present tense.
We use the present simple for something scheduled or arranged:
We have a lesson next Monday.
The train arrives at 6.30 in the
The holidays start next week.
> We use will to talk about the future:
When we make predictions:
It will be a nice day tomorrow.
I think Brazil will win the World Cup.
To mean want to or be willing to:
I hope you will come to my party. George says he will help us.
To make offers and promises:
I'll see you tomorrow. We'll send you an email.
To talk about offers and promises
To remember that you could try thinking about "will", etymologically speaking. It means "to wish"; the actual future tense built on old English is something uncertain in essence. When you say "I will do that" you meant "I wish, I hope to do that". It's how the future tense came to be. It still keep this sense in the noun form: "a will" is the expression of your wishes for after your death.
Old English willan; related to Old Saxon willian, Old Norse
vilja, Old High German wollen, Latin velle to wish, will
Compare "you will" with "you shall". and you could also read the full article on Online Etymology dictionnary
Old English *willan, wyllan "to wish, desire; be willing; be used to;
be about to" (past tense wolde), from Proto-Germanic *willjan (source
also of Old Saxon willian, Old Norse vilja, Old Frisian willa, Dutch
willen, Old High German wellan, German wollen, Gothic wiljan "to will,
wish, desire," Gothic waljan "to choose").
Compare also Old English wel "well," literally "according to one's
wish;" The use as a future auxiliary was already developing in Old
English. The implication of intention or volition distinguishes it
from shall, which expresses or implies obligation or necessity.