I seriously do not recommend shipping liquids in normal circumstances. Especially quantities greater than say 200ml. Customs can tell if a package holds liquids, there's a machine which detects this. It is very efficient at this.The reason your package could get red flagged isn't drugs, but explosives.It ought to be appreciated that the machines that detect drugs also detect explosives, that's their dual use.To your postal inspector: liquids = potential explosives. Secondly, using the pin check is business as usual. Another reason not to use liquids. That shouldn't come as a surprise to any vendor.--If after my advice, you are still determined to go ahead, then I suggest using a large vial and a small vial. The small vial contains liquid and stopped off by wax, the large vial contains sand. Finally you wrap carefully in bubble wrap and mummify it in tape. Finally, slap a fragile sticker on the package and use the return address of a scientific lab somewhere in your post code. The item should be placed within a cardboard box, and not a standard parcel package or business mailer.A water bottle is a no-no. There is no earthly reason people would post each other water bottles when they can pop down to walmart or the local shop.