Are true to size ? And do they shrink in wash?

They are a bit smaller than size. I wear a 34 to 36 size depending on the brand. I ordered Medium and they were a bit tight from the beginning, but haven't seemed to shrink. I would have been better off ordering a Large. I ordered a different brand in 34" at the same time and they fit perfect. I haven't gained weight or inches in the last 2 years so these are definitely a bit smaller than true size for guys in my waist size range. If they do shrink from drying over the next few months I'll have to throw them away.

Are there seams on the back of the boxer briefs?

No. The seam runs from the inner thigh of one leg to the inner thigh of the other leg. I would not recommend this item though as one of the 4 pair in the package I ordered fell apart after 4 or 5 wearings.