I'll take it.
'Lo prendo' — quick and decisive. 'LO PREN-do'. The 'lo' is unstressed, clitic pronoun.
The simplest way to say you're buying the item. Clear, direct, no ambiguity. Staff will then lead you to the till.
'Lo' is a direct object pronoun replacing the masculine noun (item). Feminine item: 'La prendo'. Plural: 'Li prendo' (m) or 'Le prendo' (f).
La prendo.
I'll take it. (feminine item)
Use for feminine nouns: gonna, borsa, camicia
Li prendo entrambi.
I'll take both of them.
When choosing between two items and taking both
Prendo questo.
I'll take this one.
Pointing at the item — more emphatic
Once you say 'lo prendo' in a small Italian shop, it's considered a commitment. Changing your mind at the till is acceptable but might require an explanation, especially in boutiques.