You can email invoice that has the link to pay online or auto-draft customer's bank (free) or credit card (3%-4% fee).

On a $100k property, it could cost $10k to sell it including all fees. 

I would have gladly paid the extra meter and impact fees to be provided an additional meter but that was not possible.

.$70,000 Purchase price $30,000 Rehab estimate $2,500 Purchase Closing Costs $2,700 HML Points $2,500 Refinance Closing Costs/Fees $ ????

It will usually cost you $500 in legal fees and around $200 in court filing fees.

Should I require an upfront fee, a contract, etc.

I also put lease break fees into my lease for situations like this but if the sisters are willing to cover the whole month, you may luck out and not lose any money or have to go to court at all. 

@Wayne BraultI know some brokers offer a flat fee listing on the MLS.

However if I factor in property management cost and gardener fee, I am in the hole closer to $300 a month.

In North Carolina, it's customary to have a "due diligence" fee when executing a purchase agreement.