Product weight should be set for all products,for which shipping required. Product weight parameter allows to separate all products to tangible (shipping required) and intangible (digital products, which don’t need to ship). Until your products doesn’t have weight they treat as intangible and no shipping methods (carrier-calculated, user-defined and flat rate) apply to them.
Once you have set weight for all of your products, shipping flat rate will be applied as expected.
Maria Nikiforova, Ecwid Team
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