The affix ṭhak comes after a word in the Genitive -6th-Case in construction in the sense of 'its usage'.,
The word धर्म्यं means 'usual,' 'relating to custom'. The word तस्य shows the case of the word. Thus शुल्कशालाया धर्म्यं = शौल्कशालिकः 'the custom-house laws or usages.' So also आकरिकम्, आपणिकम्, गौल्मिकम् ॥,