Whatever the case, make sure to invoice your clients consistently. You're more likely to get paid on time when your client receives invoices regularly on the first day of each month, than getting them sporadically.
Sales Tax: Depending on your jurisdiction and type of business, the tax requirement can get complicated. Be sure to follow instructions from your local tax bureau and include the appropriate tax amount on the invoice. Pro tip: Use an invoicing software to manage your business activities, change tax settings as you want and make your tax time easier.