How to satisfy a part testing contract

No, there is no explicit bonus for completing more than one contract in the same flight. However, trying to do so can often safe you some time and money. Some test contracts are really only profitable when you do them as secondary goals on a flight which also has other purposes.

Do not hesitate to decline contracts which don't earn you enough to justify a launch and don't fit together well with other contracts you already accepted. When you decline a contract before accepting it, it will be replaced with a new one which might be easier to fulfill.

However, you should avoid declining contracts, because this will give you a reputation penalty. When you don't have any interesting contracts in the command center, wait a few days until contracts time out and are replaced with new ones. Also, the game notices which kind of contracts you accept and will then give you more of these. So you might want to accept some low-risk-low-reward contracts from your favorite category just so you can get some more lucrative ones.

Contracts have a penalty for rejecting and a penalty for failing. The first is incurred when rejecting a contract before accepting it. So it is usually a better idea to wait a few (in-game) days until the contracts time out and are replaced with new ones. There is no reputation penalty when unaccepted contracts time out.

An already accepted contract is failed and will make you pay the fail-penalty if:

• You reject it after accepting it
• The time limit is exceeded (it's usually over a year, so this should only be a concern when you do an interplanetary mission)
• For rescue missions, the mission is also failed when you kill the Kerbal you were supposed to rescue.
• For tourist missions, the mission is also failed when you kill one of the tourists or expose them to so much acceleration that they pass out (if g-force is enabled in your game settings).