1. According to multiple references « faire du lèche-vitrines » is invariable with the plural form of 'vitrines'. However, it appears that the 1990 rectifications accept the singular form. The plural form is still correct but is being red-lined through the 's'.
https://www.collinsdictionary.com/dictionary/english-french/window-shopping
https://www.larousse.fr/dictionnaires/francais-anglais/l%c3%a8che-vitrines/46459
https://www.dictionnaire-academie.fr/article/A9I1911
2. The hint for "Some of them deliver to your door " is to use the plural form of "yours" - I think this is meant to be use the polite form, as the expected script is « votre porte », not « vos portes »
