Package filled and/or fillable with a pourable product comprising a main body having a designated pour opening and being formed from a multilayer composite packaging material, and an opening device (4, 4'); wherein the opening device (4, 4') is coupled onto the packaging material of the main body and about the designated pour opening; wherein the opening device (4, 4') is molded to the main body; wherein the opening device (4, 4') comprises: - a base frame configured to couple the opening device (4, 4') on the packaging material and about the designated pour opening; - a collar integrally connected to and extending from the base frame, delimiting a flow channel for the pourable product, and having an inlet and a pouring outlet; and - a closing element arranged and/or being arrangeable within the flow channel and being configured to cover and/or close the designated pour opening; wherein the closing element comprises a coupling membrane that is detachably fixed to an initial contact portion of an inner wall of the collar; wherein the collar comprises a main portion comprising the pouring outlet and extending from the initial contact portion to the pouring outlet; wherein the base frame comprises a first contact surface configured to contact the packaging material from a first face and a second contact surface configured to contact the packaging material from a second face; characterized in that the collar further comprises a sealing ring comprising the inlet and extending from the initial contact portion to the inlet; wherein the sealing ring comprises a first inner surface facing the flow channel and an outer surface opposite to the first inner surface; wherein the base frame comprises a second inner surface facing the outer surface of the sealing ring; wherein the initial contact portion is interposed between the first contact surface and the pouring outlet; wherein a thickness of an annular interface interposed between the base frame and the collar is larger than a distance between the first contact surface and the second contact surface; wherein the collar comprises an outer surface opposite to the inner wall of the collar and having a curved portion; wherein the base frame comprises a first frame having the first contact surface and a second frame having the second contact surface; wherein the first frame comprises an external surface opposite to the first contact surface; wherein the curved portion contacts and/or intersects with external surface; wherein the thickness of the annular interface corresponds to a minimum distance between the second inner surface and the curved portion. wherein the opening device (4, 4') is coupled onto the packaging material of the main body and about the designated pour opening; wherein the opening device (4, 4') is molded to the main body; wherein the opening device (4, 4') comprises: - a base frame configured to couple the opening device (4, 4') on the packaging material and about the designated pour opening; - a collar integrally connected to and extending from the base frame, delimiting a flow channel for the pourable product, and having an inlet and a pouring outlet; and - a closing element arranged and/or being arrangeable within the flow channel and being configured to cover and/or close the designated pour opening; - a base frame configured to couple the opening device (4, 4') on the packaging material and about the designated pour opening; - a collar integrally connected to and extending from the base frame, delimiting a flow channel for the pourable product, and having an inlet and a pouring outlet; and - a closing element arranged and/or being arrangeable within the flow channel and being configured to cover and/or close the designated pour opening; wherein the closing element comprises a coupling membrane that is detachably fixed to an initial contact portion of an inner wall of the collar; wherein the collar comprises a main portion comprising the pouring outlet and extending from the initial contact portion to the pouring outlet; wherein the base frame comprises a first contact surface configured to contact the packaging material from a first face and a second contact surface configured to contact the packaging material from a second face; characterized in that the collar further comprises a sealing ring comprising the inlet and extending from the initial contact portion to the inlet; wherein the sealing ring comprises a first inner surface facing the flow channel and an outer surface opposite to the first inner surface; wherein the base frame comprises a second inner surface facing the outer surface of the sealing ring; wherein the initial contact portion is interposed between the first contact surface and the pouring outlet; wherein a thickness of an annular interface interposed between the base frame and the collar is larger than a distance between the first contact surface and the second contact surface; wherein the collar comprises an outer surface opposite to the inner wall of the collar and having a curved portion; wherein the base frame comprises a first frame having the first contact surface and a second frame having the second contact surface; wherein the first frame comprises an external surface opposite to the first contact surface; wherein the curved portion contacts and/or intersects with external surface; wherein the thickness of the annular interface corresponds to a minimum distance between the second inner surface and the curved portion.