yup, the docs are wrong, do it like this:
https://{your_business}.
lessaccounting.com/proposals/11/line_items.json?lin
e_item[unit_price]=7.32&line_item[line]=yo,
dis is da ting&line_item[sales_tax_ids][]=1&line_item[sales_tax_ids]
[]=2&api_key=89341988-6b36-4128-
b3d5-deab427132a7
Notice:
1. extra brackets:
line_item[sales_tax_ids]***[]***
2. repeat the param for each id:
line_item[sales_tax_ids][]=1&line_item[sales_tax_ids][]=2
The new docs will be uploaded shortly.