No they do not have a fly opening.

BAMBOO COOL メンズ下着ボクサーブリーフソフトで快適なバンブービスコース下着トランクス (4パック) US サイズ:

BAMBOO COOL メンズ下着ボクサーブリーフソフトで快適なバンブービスコース下着トランクス (4パック) US サイズ:

Q: 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.

Q: 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.