haven't kept the receipt vs didn't keep the receipt

[vectra]

Hello,

Which of the two options - 'haven't kept the receipt' or 'didn't keep the receipt' sound more natural in the following text:

There is one more problem. I can't enclose the copy of my receipt because I paid cash and didn't keep the receipt.

I can return the faulty alarm clock in the original packaging, but I'm afraid I didn't keep the receipt.

These are sentences from my students' module test they took today. They had to compose a complaint letter. My answer is
'I haven't kept the receipt.' The result of this carelessness is that I do not have it now.

Thank you in advance.

 

[Raymott]

Hello,

Which of the two options - 'haven't kept the receipt' or 'didn't keep the receipt' sound more natural in the following text:

There is one more problem. I can't enclose the copy of my receipt because I paid cash and didn't keep the receipt.

I can return the faulty alarm clock in the original packaging, but I'm afraid I didn't keep the receipt.

These are sentences from my students' module test they took today. They had to compose a complaint letter. My answer is
'I haven't kept the receipt.' The result of this carelessness is that I do not have it now.

Thank you in advance.

I'd use "didn't" in both cases, even though not having kept the receipt is obviously relevant in the present.
In the first sentence, there is a parallelism in tenses between, "I paid cash" and "I didn't keep the receipt" which is worth keeping. In the second you could use either.