You gotta pair it with "no card needed" otherwise it sounds suss.

There's other ways. "Upgrade to a paid account if you like it" works. "Upgrade" implies work. "Paid" implies the work is paying. Thus the thing you're currently engaging with doesn't need payment.

You can also call it "Free tier" which has "14 day expiration" as a stipulation.

There's no best practice here because it's dependent on the product, value proposition, where you are in the funnel and who the customer is. They'll perform differently and counterintuitively and most frustratingly, change over time (because the context of the relationship between your prospect and product is always dynamic)

The truths that do hold are,

1. Make it sound casual

2. Avoid hustle language

3. Give an easy exit strategy

4. Make it clear and direct