The expression that best completes the sentence “If the company had any delay at the customs office, it was … their own making” is of.

This phrase indicates that if the company experienced any delay at the customs office, it was due to their own actions or decisions.

Here’s why the other options are not correct:

• at: This term typically refers to a location or point in time, and doesn’t fit well in this context.
• from: This term typically refers to the origin or source of something, and doesn’t fit well in this context.
• with: This term typically refers to being accompanied by something, and doesn’t fit well in this context.
• through: This term typically refers to moving in one side and out of the other side of something, and doesn’t fit well in this context.