Я редко ношу каблуки, поэтому летом мне удобнее ходить в сандалиях.

It is accusative plural, because носить takes a direct object: you wear something.

The reason it looks the same as the basic plural form is that каблуки is an inanimate noun. In Russian, for inanimate nouns, accusative plural = nominative plural.

So:

• каблуки = nominative plural
• ношу каблуки = accusative plural, same form

Because носить means to wear in a general or habitual sense.

• носить = to wear regularly / as a general habit
• надеть / надела / надеваю = to put on

So:

• Я редко ношу каблуки = I rarely wear heels
• Я надела каблуки = I put on heels

Also, learners often mix up надеть and одеть. In standard Russian:

• надеть = put on yourself / put on an item
• одеть = dress someone

That is the most neutral and natural word order. Adverbs like редко, часто, обычно, иногда often go before the verb:

• Я редко ношу...
• Он часто читает...
• Мы обычно ходим...

Other word orders are possible, but they usually add emphasis or sound less neutral.

Поэтому means therefore, so, or that’s why.

It introduces the result of the first clause:

• Я редко ношу каблуки → cause
• поэтому летом мне удобнее ходить в сандалиях → result

The comma is there because Russian is joining two clauses in a cause-result relationship. So the punctuation is normal here.

Летом is an adverbial use of the instrumental case and means in summer.

Russian often uses the seasons this way:

• летом = in summer
• зимой = in winter
• весной = in spring
• осенью = in autumn

So this is a very common pattern, and no preposition is needed.

Because Russian often expresses feelings, comfort, ease, and similar ideas with a dative experiencer:

• мне удобно = it is comfortable for me
• мне удобнее = it is more comfortable for me

This is different from English, where we often say I am more comfortable. Russian instead says something closer to To me, it is more comfortable.

Я удобнее would not mean the same thing. It would sound like I am more convenient/comfortable, which is not the intended idea here.

Удобнее is the comparative form meaning more comfortable or more convenient.

It comes from удобный / удобно. In this sentence it works like a predicative word:

• удобно = comfortable
• удобнее = more comfortable

The comparison is implied. The sentence means that sandals are more comfortable than heels, even though the second item is not repeated explicitly.

Because ходить is used for general, repeated, habitual, or non-one-time movement, while идти usually refers to one specific movement in one direction.

Here the speaker is talking about a general preference in summer, not one particular trip:

• мне удобнее ходить в сандалиях = it’s more comfortable for me to go around / walk in sandals
• идти would sound more like one specific occasion

So ходить is the natural choice.

Сандалиях is prepositional plural after в.

With clothes and footwear, Russian often uses в + prepositional to mean wearing something:

• ходить в пальто = to go around in a coat
• быть в кроссовках = to be in sneakers
• ходить в сандалиях = to go around in sandals

So в сандалиях does not literally focus on being physically inside them. It is simply the normal idiom for being dressed or shod in something.

Also, сандалии is normally used in the plural, just like English sandals, because it refers to a pair.