Quick answer
Я возьму́ э́то.
ya vazmú éta — perfective future, because it is one decision
The options
Я возьму́ э́то.I’ll take this.
Беру́!I’ll take it! — on the spot
Да, э́то подойдёт.Yes, that will do.
Два, пожа́луйста.Two, please.
Мне ну́жно поду́мать.I need to think about it.
возьму́ or беру́?
Возьму́ is the perfective future of взять — one decision, now. Беру́ is the imperfective present used as a snap decision, exactly like English “I’m taking it”. Both are natural; беру́ sounds a shade more decisive.
Finishing the purchase
Ско́лько с меня́? — how much do I owe? Мо́жно ка́ртой? — can I pay by card? Паке́т не ну́жен — I don’t need a bag. And at the end, Чек, пожа́луйста.
What you hear back
Хорошо́, с вас две ты́сячи.Right, that’s two thousand.
Паке́т ну́жен?Do you need a bag?
Опла́та зде́сь.Payment here.
Спаси́бо за поку́пку!Thanks for your purchase!
Typical mistakes
Я бу́ду брать э́то.
Я возьму́ э́то.
A single decision takes the perfective.
Я возьму́ э́та
Я возьму́ э́то
э́то is the neutral form used for an unnamed object.
Check yourself
Which is the natural way to close a purchase?
One decision → perfective future.