英文句子結構問題,求分析?

1. 加逗號，因為這是一個非限制性的關係從句。As "where traders make ..." is a non-restrictive relative clause, a comma must be added.

2. "full of products" is a reduction of relative clause "that are full of products" with "that are" omitted.
It becomes an adjective phrase modifying the noun "trolleys".

加返個with會唔會好D?
No, the grammar will be wrong if you add a preposition "with" in front of "full of products".
Preposition must be followed by a noun or noun phrase.

If you insist to use "with", the sentence should be rewritten as:
~ where traders make hundreds of trips a day pushing trolleys with full "loads" of products such as infant milk formula, ...

(where "loads" is a noun in plural)

1. 因為佢講到明係the Shenzhen side of the Lok Ma Chau-Futian border checkpoint, 所以就要加逗號.
如果係i still remember the place where we met. 無講明係邊度, 就5洗

2. 5洗加with, 因為已有full of ..