Learn Farsi: من این کالا را حتماً توصیه می‌کنم.

That symbol is an Arabic marker called tanvin. It adds an "an" sound to the end of a noun or adjective, turning it into an adverb. Here, it attaches to the root حتم (certainty) to create the written adverb حتماً (hatman), meaning "certainly" or "definitely".

Why does the adverb حتماً come after the direct object marker را?

The word را must attach directly to the definite object phrase it marks, which is این کالا (this product). Adverbs like حتماً are flexible in Farsi, but placing it right before the verb phrase توصیه می‌کنم is the most natural way to emphasize how strongly you recommend the item.

Since می‌کنم means "I do", what does توصیه mean on its own?

توصیه (tosiyeh) is a noun meaning "recommendation" or "advice". Farsi builds many compound verbs by pairing a noun with a helper verb like کردن (to do). So توصیه می‌کنم literally means "I do a recommendation", which we translate as "I recommend".

Could I just use the word چیز (thing) instead of کالا (product) here?

You could grammatically say این چیز (this thing), but کالا (kâlâ) specifically means "product", "goods", or "merchandise". If you are writing a review on a shopping website, کالا is exactly the right vocabulary word to use.