There are hundreds of sites providing complicated guides and advice to business. You will also find thousands of ebooks on the subject, and these are all for free, for starters. I have read a lot, one of the most profound books ever written will always be Good to Great. I don't think you can beat the basic principles discussed, even if you are a small company or mom and pop shop.
But if you had to nail a principle down to once sentence, something you can remember while doing business to help you approach customers and suppliers it will be the following:
Choose only two: price, quality or service
You can only have two, that applies for a custom that is asking for a quote or a supplier providing a quote. Decide for yourself what are you willing to give. You cannot provide all three. If you do that I promise you, you will run out of business very quickly.
When dealing with a customer you will quickly get a gut feel that they maybe after cheap products and good service, that's ok. With lower more affordable products you can raise your margin easily to pay for the good service or make servicing your customer worth your while.
People looking for quality and service will have to pay the price. You need to incorporate your efforts into the price of the product so that your time is paid for upfront. It's a risk the customer is taking, not you.
I base my business on quality and service. I do not budge on price, only when bulk is involved. But I stick with good margins that are reasonable. If I am pushed for more discounts I walk away and spend more time with my customers that are willing to pay for my service and quality.
Thank you Pixabay for the public domain image. The link for the featured image is: https://pixabay.com/en/businessman-consulting-business-2606509/