Our Prophet (Sallallahu Alaihi wa Sallam) said don't combine two transactions into one. Does this include asking the supplier to pack the customers goods and sent it as well.
I read that sheikh Ibn Uthaimeen (May Allah forgive him) said that stipulating a condition to return unsold items to the supplier is not valid.
Please forgive me for any mistakes or incorrect statements.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. If you mean that the supplier makes a sale agreement with the customers without the goods being under his possession, and then sends to you to provide him with the goods- if that supplier is an agent working for you, then it is ok. But if, on the other hand, he is not an agent who makes the sale separately for his own account then he buys the goods, then this is not allowed, for this is selling things that are out of his possession. At-Tirmithi , An-Nasa'e and Ahmad reported Hakim Ibn Hizam as having said: I came to the Prophet (may Allah's peace and blessing be upon him) and said: 'a man asked me about selling things that are not under my possession, shall I buy things from the market then sell them to him?' He (peace be upon him) said: 'do not sell that which is not under your possession.' In both cases, this procedure is not considered as combining two sales into one. As for setting a condition that the unsold goods should be returned back, it is an invalid condition, for it contradicts the validity of the contract. In this case, the condition will be invalid but the trade is sound. This was confirmed by a group of scholars. The author of "Ulin-Nuha fi Sharh Ghayatil Muntaha" stated: The second type of invalid conditions in trade is that which is invalid by itself but not causing the trade to be invalid as a condition in the contract that contradicts its essence, which is the sale dealing. An example of this is when a buyer sets a condition that he should not lose, or he will return back the goods in case it will not be sold. So, Sheikh Ibn Uthaimeen (may Allah have mercy on him) was in conformity with other scholars on this matter and did not contradict them. Allah knows best.