Shipping amount incorrect when hitting "Go back" on checkout
Check out Per Product Shipping Buggy

Import this rule or a variant of it and adapt it to your use case:

{ "commerce_shipping_service_free_shipping" : {
"LABEL" : "Rate Free shipping",
"PLUGIN" : "rule",
"OWNER" : "rules",
"REQUIRES" : [ "commerce_shipping" ],
"USES VARIABLES" : { "commerce_order" : { "label" : "Order", "type" : "commerce_order" } },
"DO" : [
{ "commerce_shipping_delete_shipping_line_items" : { "commerce_order" : [ "commerce_order" ] } },
{ "commerce_shipping_service_rate_order" : {
"shipping_service_name" : "free_shipping",
"commerce_order" : [ "commerce-order" ]
}
}
]
}
}