"... I've found with post staff here, that there can be quite a different culture in different post offices, and some will interpret the same set of rules in a different way...."
Yes, that's true here in the US as well. Last year I spent about $4000 on shipping, mostly using the USPS priority mail service.
One particular post office employee was charging my customers for additional postage. I have a commercial account with USPS and the rules say that any additional charges are to be billed to the sender (me) not the receiver (my customer). But the postal worker charged the customer anyway ... and only because that one particular PO person decided to interpret a certain set of rules in a very creative way. Thank goodness my local Post Master is a wizard for solving problems -- when I asked her for help, she took the matter firmly in hand, figured out what was happening, contacted the Post Master in the other PO about the employee, and worked with me to ensure the problem doesn't happen again. So shipping carriers -- USPS, FedEx, UPS, etc. -- all have their problems and their problem employees, but they also have a lot of good folks working for them too.
But as far as the OP's problem -- There is no need to lie. And there is no need to go to another Post Office. Soap is NOT a hazardous material. When the PO employee asks you whether you are shipping hazardous materials, have the confidence in yourself to just say "no". No need to explain the gory details, no need to worry about being arrested at the door, no need to choose your words creatively. A simple, truthful one-word reply is all that's required.
Now when I see sellers turning Priority mail boxes inside out to use for First Class mail packages, or using Priority mail padded envelopes for stuffing boxes sent via UPS ground, or other misuse of USPS mailing supplies -- that steams me. These supplies are included as part of the priority mail service, not for any other purpose, and my shipping rates are higher due to the misuse by others.