Posting and commerce options - is code injection a solution?

Site URL: https://www.bodyhappyorg.com/

Hi there, I am hoping that someone with more code experience than me could provide me with some ideas. I am a VA with a small amount of code experience. I have two clients using Squarespace who have both recently added products to their site. I am having a bit of a nightmare with the Shipping options. They are both UK based and for various reasons cannot set standard shipping rates across the board. I understand from spending a long time researching that product specific shipping is currently not an option. I have managed to make a workaround for one of the sites by setting the product weights to a certain amounts (that relate nothing to what they actually weigh) and then by setting shipping rates per weight. This seems like such a long winded, annoying and not efficient way to do it? Does anyone have any code that I can inject into the product pages in order to tweak this somehow. I also wanted to set the amount that could be ordered of one product, but again, doesn't seem to be a standard option and only something that you can potentially do by injecting code. Does anyone have any ideas? Or places to point me to? Thanks so much!