Technically, a "coffin" only applies to this shape of burial container (or the rounded cast-iron ones), which are not much in use any more. What most people are buried in today is a "casket" (more specifically a "jewel-box casket", since it's decorated and padded on the inside like a jewel box). However, a "casket" more generally is just a box. It doesn't have to be the thing you're buried in; in the song, it's apparently a keepsake or jewelry box and not, actually, the coffin.