Who's going to do escrow for an amount less than $5?
You can pay it upfront if the account is above Member level (and having Neutral pr positive trust). Very unlikely Member account having neutral or positive trust will scam you for $5. Only concern here I see you are not looking advertisement here, so very hard to verify.
OP, Good luck for your campaign. Will be happy to see you to do a successful campaign here instead of one more red painted newbie.
I'd rather see the work first than pay. Like I say, it's all dependent on how many users are following, engagement rate, and so on.