On Shopify, you're the one deciding how much does each item weight, even if it weights 50 grams you can set it in Shopify as it is 900g. And the customer will be able to order only 1 item for $10 shipping fee.
You gotta make the calculations and check how you want it to weight on shopify depending on how many items you want your customers to purchase.
I'm gonna try this method also, started setting up my shop earlier this morning but i'm stuck at the payment methods as i'm from Morocco, only Paypal+2Checkout/Paystack is available here, gotta wait until my Paystack account is verified i guess :/