Hey Martin,
A lot of wholesalers do have a weight chart, have you emailed them to see if they have more information there? The options with Shopify are either using weight based shipping or price based (so the total price of the entire cart of items).
If you have an idea of the weight of some things and are wary of setting a flat rate up for everything, consider the app Better Shipping: https://apps.shopify.com/better-shipping as that app lets you set specific rates for items as you determine the range you need.
Let me know if you have questions!
-Alex M.
Shopify Guru
alex.missett@shopify.com