kono apuri ha benride, sikamo yasui desu.

は is the topic marker. It tells you what the sentence is about.

So in:

このアプリは便利で、しかも安いです。

the topic is このアプリ = this app.

A natural way to think about it is:

• As for this app, it’s convenient, and what’s more, it’s cheap.

It is not marking the subject in exactly the same way English does. It is setting up the topic: “speaking of this app…”

この means this, but it must be followed by a noun.

• このアプリ = this app

By contrast, これ can stand on its own:

• これは便利です。 = This is convenient.

So you use:

• この + noun
• これ by itself

That is why the sentence uses このアプリ, not just この.

アプリ is a loanword, so it is written in katakana.

It comes from application / app, and in Japanese アプリ usually means app, especially for phones or software.

Katakana is commonly used for:

• foreign loanwords
• some onomatopoeia
• emphasis
• certain technical terms

So アプリ in katakana is completely normal.

Because 便利で is the connecting form used to link one description to another.

Here, the sentence connects:

• 便利 = convenient
• 安い = cheap/inexpensive

So:

• 便利で、しかも安いです
• convenient, and moreover cheap

With a な-adjective like 便利, the connecting form is ～で.

Compare:

• 便利です。 = It is convenient.
• 便利で、安いです。 = It is convenient and cheap.

Using 便利です、しかも安いです is also possible, but 便利で makes the sentence flow more smoothly as one combined statement.

Because な is only used when a な-adjective directly modifies a noun.

For example:

• 便利なアプリ = a convenient app

But in your sentence, 便利 is not directly modifying a noun. It is being used as part of the predicate and then connected to the next idea. So you use:

• 便利で

not:

• 便利な

So:

• 便利なアプリ = convenient app
• アプリは便利です = the app is convenient
• アプリは便利で、安いです = the app is convenient and cheap

しかも means something like:

• moreover
• what’s more
• on top of that
• and to top it off

It adds another point, often with a feeling that the second point is especially good, surprising, or persuasive.

So:

• このアプリは便利で、しかも安いです。

has the feeling of:

• This app is convenient, and what’s more, it’s cheap.

It is stronger and more emphatic than a simple and.

They are similar, but not identical.

• そして = and / and then
  A more neutral connector.

• それに = besides / in addition
  Adds another supporting point.

• しかも = moreover / what’s more / on top of that
  Adds a point with extra emphasis, often suggesting “and that’s not all.”

In this sentence, しかも works well because cheap is being presented as an especially appealing extra benefit.

So the nuance is a bit like:

• It’s convenient. And on top of that, it’s cheap.

Because 安い is an い-adjective, and in polite present form you normally say:

• 安いです

not:

• 安くです

For い-adjectives:

• dictionary/plain form: 安い
• polite form: 安いです
• connecting form: 安くて

Examples:

• このアプリは安いです。 = This app is cheap.
• このアプリは安くて便利です。 = This app is cheap and convenient.

So in your sentence, 安いです is correct because it is the final predicate of the sentence.

In Japanese, when you connect multiple descriptions in one sentence, the politeness marker です often appears only at the end.

So:

• 便利で、しかも安いです

is natural and polite.

You do not need:

• 便利です、しかも安いです

unless you want two more separate clauses.

Japanese often saves the final です / ます for the end of the sentence, especially when ideas are linked together.

It is polite because it ends with です.

• このアプリは便利で、しかも安いです。 = polite
• このアプリは便利で、しかも安い。 = casual/plain

Even though 便利で itself does not contain です, the sentence as a whole is still polite because the final predicate is polite.

Yes, that is grammatically fine.

For example:

• このアプリは安くて、しかも便利です。

This means the same basic thing, but the emphasis changes slightly. The point before しかも comes first, and the point after しかも may feel like the stronger extra selling point.

So:

• 便利で、しかも安いです
  = It’s convenient, and on top of that, it’s cheap.

• 安くて、しかも便利です
  = It’s cheap, and on top of that, it’s convenient.

Both are natural; they just highlight different things.

Not with 便利.

That is because 便利 is a な-adjective, not an い-adjective.

• な-adjective connection: ～で
• い-adjective connection: ～くて

So:

• 便利で = correct
• 安くて = correct

But:

• 便利て = incorrect
• 安いで = incorrect

This is a very common pattern learners need to memorize.

No, the comma is not strictly necessary, but it is very common and helps readability.

You may see:

• このアプリは便利でしかも安いです。
• このアプリは便利で、しかも安いです。

The version with the comma is easier to read because しかも is a discourse connector and the pause feels natural.

So the comma is mainly a writing/style choice here.