kounyuusya ha daikin dake de naku, souryou mo harau.

購入者 is a fairly formal noun meaning buyer / purchaser.

It is made of:

• 購入 = purchase
• 者 = person

So it literally means the person who purchases. This sounds more businesslike or written than everyday expressions like 買う人.

は marks 購入者 as the topic of the sentence.

So the structure is roughly:

• 購入者は = as for the buyer
• 代金だけでなく、送料も払う = pays not only the price, but also the shipping fee

This is a very common Japanese pattern: topic + comment.

だけでなく means not only ....

In this sentence, it pairs with も later:

• 代金だけでなく、送料も = not only the price, but also the shipping fee

A very useful pattern to memorize is:

X だけでなく、Y も ...

= not only X, but also Y ...

In this expression, it is best to learn だけでなく as a set phrase.

It is related to the fuller form だけではなく:

• だけではなく → more full/formal
• だけでなく → shorter standard form
• だけじゃなくて → more casual spoken form

So the で here is not the usual で meaning at / by / with. It is part of the grammar pattern.

も means also / too here.

It matches the earlier だけでなく:

• 代金だけでなく = not only the price
• 送料も = the shipping fee too / also the shipping fee

So も is what gives the also part of not only A, but also B.

This is a very common thing in Japanese.

With 送料も, the も effectively takes the place of を. In other words, 送料も払う means pay the shipping fee too.

With 代金だけでなく, Japanese often leaves the case marking implicit in this pattern. The object relationship is still understood from the verb 払う.

So even though both 代金 and 送料 are things being paid, Japanese does not need to show を in the same way English needs an explicit object structure.

払う is the plain, dictionary-form version of the verb.

This does not automatically mean the sentence is rude. It just means the sentence is written in a plain style, which is common in:

• explanations
• written statements
• rules and policies
• textbook example sentences

Also, Japanese non-past forms like 払う can express:

• a habitual action
• a general rule
• a future action, depending on context

Here it sounds like a general statement or rule about buyers.

Yes.

• だけでなく = more neutral, standard, somewhat formal
• だけじゃなくて = more casual, conversational

So in a businesslike or written sentence such as this one, だけでなく fits very naturally.

The comma helps separate the two halves of the pattern:

• 代金だけでなく、
• 送料も払う

It makes the sentence easier to read. In many cases, the comma is optional, but it is very common in writing when the sentence has a contrast/addition pattern like not only X, but also Y.

They refer to two different kinds of money:

• 代金 = the price/payment for the item itself
• 送料 = shipping fee / postage / delivery charge

So the sentence is distinguishing between the cost of the product and the extra cost of sending it.