In the UK the big risk is sales tax (VAT). It needs forecasting on top of sales receipts, and every quarter you have to pay this sum you have collected over to the exchequer. VAT is notorious for taking down businesses who spent the receipts!

Otherwise you should model what happens when the sales come late or not at the level you want, Braintree hold your cash, etc. If you can't flex your overheads to stay within your cash facilities, then you are risking insolvency.

Sales receipts, payroll and sales tax are the big numbers.