"And oblige" adds nothing to the letter. If the reader is not already influenced by its contents, "and oblige" will not induce him to be.
The telegraphic brevity caused by omitting pronouns and all words not necessary to the sense makes for discourtesy and brusqueness, as:
Answering yours of the 21st inst., order has been delayed, but will ship goods at once.
How much better to say:
We have your letter of 21st October concerning the delay in filling your order. We greatly regret the delay, but we can now ship the goods at once.
"Same" is not a pronoun. It is used as such in legal documents, but it is incorrect to employ it in business letters as other than an adjective. Use instead "they," "them," or "it."
Incorrect:
We have received your order and same will be forwarded.
Correct:
We have received your order and it will be forwarded.