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Companies have lawyers. Legal needs to sign off before some big purchases can be made. You will need a software license agreement. Purchase one of these for a few grand. Best to do it upfront.
Give quotes that last for 60 days. Companies will cut you a purchase order and expect you to ship the software. The game is not over yet. You still need to collect. Luckily the purchase order is legally binding. It will just take a while for you to get your money.
Get to know the payment procedure for each of your big company customers. It will make your payments come quicker. Know this. Companies don't like to pay via PayPal. Make it easy for them to pay you by whatever means necessary, except for Monopoly money.