CALA uses a weight based pricing model you can view here to determine the shipping cost of each item.
You will have the option of either charging your customers the same rate, or setting a simple flat fee (example: $5 Domestic, $15 International). We can help you load the pricing into Shopify, just let us know which method you prefer.
Then for payment, CALA fulfillment uses a pay-as-you ship model. You'll be invoiced by CALA twice a month for the fulfillment from the previous 2 weeks.