Well, you could have told him "It's my laundry" and ignored him. Or better yet, explained that it's your laundry, shown him the bill and then walked off. Or, though it's less reasonable on your part, just ignored him and walked off. Even police officers have specific criteria they have to meet before they can arrest someone, and non-officers are MUCH more restrained in their ability to arrest. And as the items weren't stolen, you cannot have committed an arrestable offence, which means that if he arrests you, the arrest is retrospectively unlawful when it turns out the items you "stole" weren't stolen, so no arrestable offence has been committed, so you've been assaulted and unlawfully imprisoned if physically detained.
If he physically restrains you, it's either a citizen's arrest or he's assaulted you. In fact, strictly speaking, he can assault you without even physically touching you, because assault is more about credible fear of physical harm, so even intimidating you into complying (which is not the same, of course, as just asking) would be an assault, and it's an arrest if he tells you your arrested without even touching you. And if he arrests you, and gets it wrong (which it would be if the suit wasn't stolen), sue him AND the store .... and have him charged with assault or some form of battery if he used force, because that reasonable force used to arrest you is assault if the arrest is not valid.
And, of course, if the arrest was invalid, and you used force to resist his illegal arrest, THAT would be legal, provided it was reasonable force.
If he stops you by
asking you to stop, and asking you to accompany him, and you do, then you've gone voluntarily. But if you don't go voluntarily, about all he can do is either let you walk away, or physically detail you while waiting for the police. And when they get there and it turns out he's "arrested" you for leaving with your own laundry ....
Store staff have to be VERY careful arresting people. But asking you to stop and accompany them is not an arrest, if you comply willingly.