You don't say where you are located... I am only familiar with US laws regarding direct marketing product to consumers.
Back in the 70s. the FTC tightened up the rules such that you must ship a mail order sale within 30 days of receiving payment, or mail a notice to the customer offering their money back if the shipment was still delayed. Back in the 70s we didn't have email and my kit business would have boom bust cycles where I could sell hundreds of kits when the Popular Electronics article hit,,, Operating a one man business I would sometimes have to consider stopping building kits to notify customers I was delayed... :
By the late 70s I discovered a slick mailer available with pin feed that used carbon paper inside, so I could address the mailer and print a short message using my sturdy dot matrix printer.
Of course times have changed and customer now expect near instant gratification. Years ago I used to schedule my shipments for once a week, now customer orders get shipped often within 24 hours.
Modern electronic payment methods also offer more flexibility. I used to accept credit cards before dropping them because of the large overhead fees. I still take Paypal which is lower overhead and pretty widely used, years ago people would not buy using Paypal, now I do not get complaints and people can use paypal with credit cards.
With the credit card service I could authorize a card transaction but not capture the funds until after I shipped. I don't think Paypal has that feature.
If you are selling from a website you can incorporate a paypal "buy now" button to collect a suitable deposit for an order. I do not know what is suitable for your product or customers. Then when you are ready to ship you can send the customer a paypal invoice for the balance.
Just be clear up front with your customers what your terms and conditions are.
It is so much easier these days than back several decades ago... good luck.
JR