If you would like to order your poster print with a matching frame, make sure to have selected one of the formats for which a frame is available. In the format selection, you'll recognize this by a small icon to the right of the corresponding size. You are then prompted to select the desired paper. You can add a frame only if you also choose the paper type "printing paper matt". Choose a border colour and a passepartout and you can order your poster in the frame.
It's even easier to add a frame by using the Pixum Photo World software. Start a new project with one of the above poster formats. In the editing mode, you will find the "Picture Frames" area in the left-hand navigation. There you can select the desired frame and simply drag it onto the editor area. In the "Passepartouts" section, you can explore and try out the effect of the different passepartouts (also known as "mounts").