Hi Kooyeen
The use of 'will' in this context would tend to suggest "I'm willing to do this at a later date".
| The meaning is practically "present" though, the same as "That's too heavy for you, I'll help you..." (I'm about to help you). |
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Your sentence (above) has a different function: It is an offer to do something. As such, it theoretically requires agreement/acceptance by the other person before you do it and therefore there is still the possibility that the future activity will not happen (i.e. the other person has the option of saying something such as "Thanks, but that isn't necessary.").
Informing the recipient about an enclosure in a letter is not an offer. At the time you're writing the letter, the enclosure is a planned part of the letter-writing activity and when the person receives the letter, the enclosure is a done deal -- there is no opportunity for the recipient to say "No, thanks."
You can look at "I'm enclosing" two different ways:
- I'm currently in the process of doing everything necessary to complete this letter and the enclosure is part of that current activity.
OR
- I've already got a definite plan to enclose something with the letter.
Both of these are standard usage of the present continuous.