Speaking as someone who has spec'ed out packaging, I think understand. When specing out packaging usually there are two things you usually consider.....oxygen permiability or water vapor. For developer, the gas/oxygen permeability is what you care about and oriented PET (like blow molded bottles) has about 1/50th of the O2 permeability of HDPE. The previous post that mentions a lower barrier number is probably PETG(ref. here HDPE is a lot cheaper to blow mold and has very good moisture barrier properties. The reason that the PET developer bottles suck in is that the oxygen gas in the bottle is going into the solution and oxidizing the developer. The bottle doesn't allow air into the bottle to replace the oxygen being consumed by the oxidizing developer so it sucks in. The HDPE wouldn't suck in because the air can more easily get through the bottle to replace the oxygen........bottom line is that PET is a good bet. Personally, I use glass because it is even better.